YES, THEY WERE PLEASED TO DO SO AND THEY ARE INDEBTED TO THEM: eudokesan (3PAAI) gar kai opheiletai eisin (3PPAI) auton: (Ro 11:17; 1 Corinthians 9:11; Galatians 6:6; Philemon 1:19)

"They" refers to the saints at Macedonia and Achaia (see  discussion at Romans 15:26)

Pleased (2106) (eudokeo from eú = well, good + dokéo = think) means literally to think well of and thus to approve of or take pleasure in. From its usual sense “to take pleasure or delight in,” a further sense evolved meaning “to decide for,” “to select,” “to prefer,” and from this evolved the sense “to want,” “to will,” “to be willing or ready.” This latter meaning is the emphasis in this passage. In reference to the charitable giving, Paul makes it clear that the Gentile's gift was a freewill offering.

The Gentile believers were pleased to give such a gift to the suffering Jewish believers in Judea. They did it voluntarily. They did it willingly, not out of complusion. They gave according to the spirit with which Paul encouraged the Corinthians to give...

Let each one do just as he has purposed in his heart; not grudgingly or under compulsion; for God loves a cheerful giver. (2Cor 9:7) (Comment: In fact the charitable giving which Paul is describing here in Romans 15 is the collection that we read about in 1 Corinthians 16:1 and 2 Corinthians 8 and 9)

Indebted (3781) (opheiletes from opheilo = owe) (Click word study on opheiletes)  ("our debtors" - see notes Matthew 6:12) describing one who owes anything to another person, especially money but also used metaphorically as here of those bound by some obligation or duty. The idea is to consider something as good and therefore worthy of choice.

Paul makes it clear that the Gentiles had a moral obligation and debt to pay for they had received the gospel from Jewish believers.

J Vernon McGee puts it this way...

Macedonia and Achaia were obligated to Jerusalem. Now some of the saints in Jerusalem were having financial difficulties, evidently because of persecution. Macedonia and Achaia could now pay a spiritual debt in the coin of the realm. This is foreign missions in reverse! It is the missionary church helping the home church. This very thing may take place in our nation, by the way, in the not too far distant future! (McGee, J V: Thru the Bible Commentary:  Nashville: Thomas Nelson)

In a similar way, Paul felt a moral obligation or debt to preach the gospel writing

"I am under obligation (opheiletes) both to Greeks and to barbarians, both to the wise and to the foolish."  (see notes Romans 1:14)

Likewise believers are under a moral obligation to mortify or put to death the deeds of the flesh (kill them or they will kill you) by the Spirit as Paul says in chapter 8 of Romans writing:

"So then, brethren, we are under obligation (opheiletes), not to the flesh, to live according to the flesh -- 13 for if you are living according to the flesh, you must die; but if by the Spirit you are putting to death the deeds of the body, you will live." (see notes Romans 8:12, 8:13)

FOR IF THE GENTILES HAVE SHARED IN THEIR SPIRITUAL THINGS THEY ARE INDEBTED TO MINISTER TO THEM ALSO IN MATERIAL THINGS:  ei gar tois pneumatikois auton ekoinonesan (3PAAI) ta ethne, opheilousin (3PPAI) kai en tois sarkikois leitourgesai (AAN) autois:

If (1487) (ei) introduces a first class conditional statement which implies that the condition had been fulfilled. Thus one could render it "Since the Gentiles have shared".

Gentiles (1484) (ethnos) is used principally for peoples not Jewish and thus depending on the context can describe Gentile, heathen, nation, people. From a Jewish perspective the phrase the nations (tá éthne), indicates Gentile nations or the Gentiles in general.

Shared (2841) (koinoneo from koinos = that which is in common, belonging to several or of which several are partakers - the verb koinoneo is related to the noun koinonia translated "contribution" in Romans 15:26 - note) means to to share one's possessions, with the implication of some kind of joint participation and mutual interest. This Greek word is used in a marriage contract where the husband and wife agree to a joint-participation in the necessaries of life. The key idea in the word is that of a partnership, a possessing things in common, a belonging in common to.

Spiritual (4152) (pneumatikos from pneuma = spirit) relates to the human spirit, as the part of man which is akin to God and serves as his instrument or organ. It refers to the nonmaterial rather than the material part of man. It is used predominately of what belongs to the supernatural world as compared to that which belongs to the natural world. Note that the suffix "-ikos" on the end of an adjective means signifies “-like”. Thus pneumatikos means “belonging to the spirit" or "of the nature of the spirit" and so "pertaining to that which is spiritual".

The spiritual things of which the Gentiles partake are the spiritual blessings of salvation, specifically the gospel truths first preached to the Gentile believers by the Jewish apostles, prophets, teachers, and evangelists. Thus the Gentiles are debtors to the Jews for as our Lord taught “Salvation is of the Jews” (Jn 4:22).

Indebted (3784) (opheilo) means to be indebted to someone in a financial sense and thus to owe something to someone. Figuratively as used in this verse it means to be under a moral obligation to meet certain expectations. It can also mean to be to be constrained by circumstance. Gentiles are debtors to the Jews.

Middletown Bible Church writes that...

Gentiles are debtors to Jews. We owe them so much because "salvation is of the Jews" (John 4:22). We owe them so much because the Bible is from the Jews. Indeed, even the authors of the New Testament were Jewish, with the only exception being Luke. We are debtors to the Jews because their God has become our God. Their Messiah has become our Saviour! Gentiles have been made partakers of their spiritual things! It is only fitting that Gentiles should minister to them in carnal (material) things. Great spiritual blessings have come from them to us. It is appropriate then that great carnal or material blessings should come to them from us. We should help our Jewish brethren in every way we can. In a way we are also debtors to unbelieving Jews because of their godly heritage, even though they are not presently representing that heritage. The believer in Christ should be the best friend the Jew has in this world! (Romans 15)

Minister (3008) (leitourgeo leitourgeo  from leitourgos = public servant from léïtos = of the people [NIDNTT says it from "laos" = people] + érgon = work) means to be a worker of the people or a public servant. In classical Greek the root word leitourgos referred especially to persons performing public duties, or works of public use and so to perform religious or charitable functions (worship, obey, relieve). It meant to render special formal service, and thus leitourgeo can literally describe a priest who officiates or  ministers.

As used in this verse leitourgeo  describes the performance of religious duties in the sense of a Christian performing services to God through such activities as prayer, teaching, etc.

Leitourgeo is used 2 other times in the NT (Acts 13:2 = "And while they were ministering to the Lord and fasting, the Holy Spirit said, "Set apart for Me Barnabas and Saul for the work to which I have called them." and Heb 10:11 = "And every priest stands daily ministering and offering time after time the same sacrifices, which can never take away sins") and 86 times in the Septuagint (LXX) (Ex 28:35, 43; 29:30; 30:20; 35:19; 39:1, 26, 32; 40:30; Num 1:50; 3:6, 31; 4:3, 9, 12, 14, 23f, 26, 30, 35, 37, 39, 41, 43; 8:22, 26; 16:9; 18:2, 6f, 21, 23; Deut 10:8; 17:12; 18:5, 7; 1 Sam 2:11, 18; 3:1; 2 Sam 19:18; 1 Ki 1:4, 15; 8:11; 19:21; 2 Ki 25:14; 1 Chr 6:32; 15:2; 16:4, 37; 23:13, 28, 32; 26:12; 27:1; 2 Chr 5:14; 8:14; 11:14; 13:10; 15:16; 17:19; 22:8; 23:6; 29:11; 31:2; 35:3; Neh 10:36; Ps 101:6; Jer 52:18; Ezek 40:46; 42:14; 43:19; 44:11f, 15ff, 19, 27; 45:4f; 46:24; Dan 7:10; Joel 1:9, 13; 2:17)

Thayer writes that leitourgeo means

"in Attic, especially the orators, "to serve the state at one's own cost; to assume an office which must be administered at one's own expense; to discharge a public office at one's own cost; to render public service to the state.... universally, to do service, to perform a work; Vulgate ministro (A. V. to minister); of the priests and Levites who were busied with the sacred rites in the tabernacle or the temple (as in the Septuagint)

Paul's use of this specific word (leitourgeo) helps understand the thought he is conveying here (e.g., this word gives us our English word "liturgy"), that is, that the Gentiles are performing a "priestly service" Thus Paul elevates the ministry of almsgiving on the footing of a sacrificial service, in every sense an act of worship to God. In sum service to God's people is service to God. Let us all keep alert to the opportunities we have to minister in this way and do so with hilarity.

As an aside, it is notable that his was not the first time a collection was taken to the church in Jerusalem. About ten years earlier, Paul and Barnabas brought a collection from the church in Antioch of Syria to help the Jerusalem church during a time of famine (Acts 11:30; 12:25).

If somebody blesses you spiritually, and the only way you can thank him is with material things, then do it, That is God's program, to give back in material things for the spiritual blessings you have received. Jesus declared that 

"It is more blessed to give than to receive." ( Acts 20:35)

John MacArthur notes that...

It was not that the Gentiles’ being spiritually and materially indebted to the Jews made their generosity any less meaningful or loving. Duty does not exclude willing and joyful compliance. Even the most demanding duty or indebtedness can be fulfilled out of love." (MacArthur, J: Romans 9-16. Chicago: Moody Press)

"Material things" is literally "fleshly things".

Material (4559) (sarkikos from sarx = flesh + ikos = as noted above this suffix means "-like" = “belonging to the sarx" or "of the nature of the sarx") means that which belongs to the nature of the flesh and so that which is from the earthly sphere of existence and which is thus material or physical, the meaning intended in this passage. In other contexts sarkikos, of the nature of the flesh, conveys an entirely different meaning, referring to the behavior which is characteristic of the fallen flesh (sarx - specifically the evil disposition), but that is clearly not the meaning in this verse.

In a similar manner Paul writes to the saints in Galatia exhorting them to...

let the one who is taught the word share all good things with him who teaches. (Gal 6:6) (Comment: "share" commonly refers to sharing of material goods. Paul is saying that the responsibility of Christians to provide financial support for those who devote full time to preaching and teaching the Word of God)

William Newell writes that in this verse...

THEREFORE, WHEN I HAVE FINISHED THIS, AND HAVE PUT MY SEAL ON THIS FRUIT OF THEIRS: touto oun epitelesas (AAPMSN) kai sphragisamenos (AMPMSN) autois ton karpon touton: (Philippians 4:17; Colossians 1:6)

Paul's desire was to take the love gift to the needy saints in Jerusalem.

Finished (2005) (epiteleo from epí = intensive, in the sense of meaning "fully" + teleo = to complete, bring not just to the end but to the destined goal from teleo in turn from télos = end, goal. Note: télos originally meant the turning point, hinge, the culminating point at which one stage ends and another begins; later the goal, the end. Marriage and death are is in this sense both a telos) conveys the intensified meaning to "fully complete" or to fully reach the intended goal, to accomplish fully or to make perfect. The historian Herodotus writes that epiteleo was used of performing of religious services and refers to the act of completing something. Epiteleo was the normal Greek word for fully completing the ritual of any sacrifice.

Epiteleo (Click word study on epiteleo) is the same verb Paul choose to describe God's work in believers in Philippi writing that he was confident...

"that He who began a good work (may refer to their salvation or their active financial participation in the furtherance of the gospel) in you will perfect it until the day of Christ Jesus (not the Day of the Lord but the day that  looks to the final salvation, reward, and glorification of believers - see summary discussion of The Three Divine Days)." (see note Philippians 1:6)
 

Paul was anxious to complete this project of the collection of money for the Jerusalem poor. But in this verse Paul gives us all a good practical principle for life: He is not going to quit until he is through. He will wrap it up well and do it right. Do you finish what you begin in the realm of spiritual matters? What about when you tell someone you will pray for them...do you "finish" that good work?

Seal (4972) (sphragizo from sphragis = seal) (cf 2Cor 1:22, Eph 1:13, Eph 4:30, 7x in Revelation) a seal signifying ownership & guarantee of the correctness of contents. To set to, is to affix. To set to a seal is therefore to attest a document. Paul secured to the Macedonian and Achaian saints the product of the contribution. In other words Paul’s mission to Jerusalem would not be finished (epiteleo) until he had successfully and safely delivered the contribution. The "fruit" (charitable gift) was sealed to them when it was safely delivered and securely in their possession. The gift had already been gathered but it had not yet been delivered.

Leon Morris has an interesting discussion of the meaning of "seal" in this verse noting that when Paul speaks of...

“having sealed to them this fruit”, the NIV understands it in the sense made sure that they have received it. This may be the way to take it, but the meaning of “sealing” in this context remains doubtful. It is known that produce such as wheat and barley was put in sacks and sealed; this was the last thing done and indicated that everything was in order. It may be that that is the way we should understand it: Paul’s seal meant that the money was there in full amount. A somewhat similar view is that which starts from the seal as a mark of personal authentication; Paul’s seal was his guarantee that all had been done well. It has been held that Paul’s very presence certified the gift and set the seal on his apostleship. Knox holds that when the money has been given and received in the right spirit “Paul will have ended his divinely appointed work in Asia Minor and Greece; the ‘fruit’ of his mission will have been ‘sealed.’ ” There are other views, and at this distance in time we cannot be sure of the precise significance of the sealing. But it is clear that in some way it pointed to official assurance that all was well. (Morris, L. The Epistle to the Romans. W. B. Eerdmans; Inter-Varsity Press)

Fruit (2590) (karpos) (Click in depth word study on karpos) refers to literal fruit but metaphorically (as in this context) refers to deeds, works or conduct. Scripture catalogs three kinds of spiritual fruit: (1) spiritual attitudes that characterize a Spirit-led believer (Gal 5:22-23); (2) righteous actions (Romans 6:22, Hebrews 13:15); and (3) new converts (Romans 16:5)

In the present context fruit speaks of the love-gift (a gift of money inspired by Christian love) from the Gentile believers of Macedonia and Achaia, given sacrificially for the poor saints in Judaea. In a real sense the love-gift was “fruit” in that it was the product of gospel seed-sowing.

In Philippians 4:17 Paul uses karpos writing...

Not that I seek the gift itself, but I seek for the profit (fruit - karpos) which increases to your account. (see note Philippians 4:17) (Comment: Here karpos is used of fruit that grows like interest in a savings account. The Philippians were in effect storing up for themselves treasure in heaven -- see note Matthew 6:20. The gifts they gave to Paul were accruing eternal dividends to their spiritual account).
 

I WILL GO ON BY WAY OF YOU TO SPAIN: apeleusomai (1SFMI) di humon eis Spanian: (Ro 15:24; Proverbs 19:21; Lamentations 3:37; James 4:13-15)

AND I KNOW THAT WHEN I COME TO YOU: oida (1SRAI) de oti erchomenos (PMPMSN) pros humas:

Know (1579) (eido, oída, perfect tense of obsolete eído) speaks of knowledge which comes from one’s state of being or of  intuitive knowledge. The idea of oida is that Paul's knowledge is absolute, positive and beyond a peradventure of a doubt.

Morris notes that...

Paul closes this part of his letter on a note of quiet certainty (I know); his certainty is one of faith, but none the less certain for that. (Ibid)

Here Paul expresses a measure of confidence that he will finally be able to fulfill his longing to visit Rome, once the Jerusalem task is completed.

I know (perfect tense conveys the idea I have come to know