You see, however, how beautifully he instructs them to frame their words when they are offering up the first fruits, that we may know that the offering is nothing else than praise and thanksgiving for deliverance from enemies and for the occupation of the land. This is no different from a renter’s acknowledgment to his landlord, by means of his rent, that he has the field through the landlord’s favor. These were the first fruits, which they offered up to the priests on the Feast of Pentecost, of which chapter 16 spoke.
So he also treats the tithes to be paid every three years, teaching that they are to be given to the Levites, the orphans, the widows, and the strangers, with the affirmation that they are a fulfillment of the work of love. “I have removed it,” he says (v. 13), not to the place of sacrifice but to a particular place in which such tithes for the poor were brought together in the individual cities, as into a common treasury or chest. “I have not transgressed any of Thy Commandments,” namely, so far as pertains to this work; otherwise who fulfills a single Commandment completely? “Nor have I eaten of them in mourning”; that is, I have not done it sadly, but joyfully; for “God loves a cheerful giver” (2 Cor. 9:7). “Nor have I set them apart for anything unclean”; that is, I have set them apart, not with sin or uncleanness but in holiness; for the offering of the unclean displeases as much as the sacrifice of the sad. “I have not offered any of it for a funeral,” which would defile both the doer and the gift of the tithe.
“This day the Lord your God commands you” (v. 16). These are words of the established covenant, that they should keep the precepts which he has commanded. And he adds the promise: “If they keep the precepts, the Lord will make them high in praise, name, and glory above all the Gentiles, as He also did as long as they kept the covenant. This is the end of the Decalog and the Commandments. Treated in the following chapters are certain ordinances to be kept at particular times, as well as exhortations, approvals, curses, blessings, and the like, for the purpose of admonishing the people to keep the Commandments of God.
Luther’s works, vol. 9: Lectures on Deuteronomy 1525