Dr. Martin Luther continues his commentary on Deuteronomy with this summary of chapter 15 – and it´s good reading about lending, cancelling of debt, poor people etc in this translation by Richard R. Caemmerer in Luther´s Works Volume 9:

The summary of this chapter is that the poor should not be left neglected in the land, although he also says that there will always be poor, in order that they may have someone to whom to give, either from the triennial tithes, from loans, or from some other source… Now let us talk about lending.

Moses says that repayment of a loan can be demanded before the seventh year, but that in the seventh year itself whatever loans will be found unpaid shall be canceled and given to the brother, and that by divine right. That this seventh year, however, was fixed and firmly established, just as the year of jubilee was, for restoring and canceling debts on purchases, follows from the fact that it does not begin from the day on which the loan was made but was general for the whole land and people, like the festivals were, Passover, Pentecost, and others. Truly it is a most beautiful and fair law. Would that today the rulers of the world might imitate it! Then they would have fewer questions and commotions; for people would know that suits, disputes, debts, dealings, agreements, judgments, seals, and letters would all be removed at one time and canceled in the seventh year, whether that be close or far away, and not be postponed and continued forever into endless litigation. Likewise they would be forced to be cautious not to lend so great a sum that there would be no hope of repayment before the seventh year. Nor would it be possible for wasteful and depraved people to rely on other people’s wealth, which they would gather through borrowing and agreements.

But what will you say to Christ, who in Matt. 6 and Luke 6:35 forbids to demand repayment of a loan and commands to lend without the hope of receiving equal value in return? I answer: Christ is speaking to Christians, who are above every law and do more than the laws ordain; but Moses provides laws for people in civil society, who are subject to the government and the sword, so that evildoers are curbed and the public peace is preserved. Here, therefore, the law is to be so administered that he who has received a loan pays it back, although a Christian would bear it with equanimity if such a law did not come to his aid and a loan were not repaid. Similarly, the law demands that one harm nobody, and it works vengeance on the violent; and yet the Christian endures it if he is harmed, and he does not avenge himself or seek vengeance, although he does not forbid the strictness of the avenging sword, since he knows that the sword was established for vengeance on evildoers, as Peter says (1 Peter 2:14).

Again, why is it that he permits repayment of a loan to be demanded from a stranger—even in the seventh year, that is, always—but not from a brother? Should justice and love not be observed also toward a stranger? The answer is that this, too, is according to a just principle of public order, that by some privilege citizens are honored beyond outsiders and strangers, lest everything be uniform and equal. Thus the Romans gave some cities the status of a colony, others that of a Roman city. The world has need of these forms, even if they appear to have a show of inequality, like the status of servants and maids or workmen and laborers. For not all can be kings, princes, senators, rich men, and freemen in the same manner, since the world cannot exist without persons of various and different sorts. While before God there is no respect of persons (Acts 10:34), but all are equal, yet in the world respect of persons and inequality is necessary. And the purpose of this is that evildoers be curbed and the public peace stand firm, which it cannot do when persons are equal and without distinction.

But the people of the Jews had a fuller and higher law, not only with regard to the repayment of loans but, as he says here (v. 6), with regard to lending to the Gentiles on interest and taking usury, namely, by divine authority, which establishes and permits this very thing. For He is God and the Lord of all; He takes away not only money and goods but also kingdoms and empires from whomever He wills and however He wills, and gives them to whomever He wills. If, therefore, for the sake of vengeance on the Gentiles, God wants to punish them through usury and lending, and commands the Jews to do this, the Jews do well obediently to yield themselves to God as instruments and to fulfill His wrath on the Gentiles through interest and usury. This is no different from when He commanded them to cast out the Amorites and the Canaanites. Thus if to God a husband seems worthy of having his wife or sons taken way from him, and through His Word I am ordered to do this, I am not an adulterer or a kidnaper when I carry off his wife and sons but an obedient whip of God over the godless husband.

This answers the question how the Jews were permitted to lend on interest. The answer is: It was not and is not permitted them because of their merit or by common law but through the wrath of God over the Gentiles, which He wants to fulfill through the Jews as instruments of His wrath. Nevertheless, they would not have had the right to use this permission unless they had been commanded and chosen as such instruments through a sure and manifest Word of God. They themselves were no better than any Gentiles, as I have said above; but God chose and received them only out of mercy. Thus if you view the matter properly, it is not the Jews themselves who are usurers, but God, who persecutes the Gentiles through the usury of the Jews. This was sufficiently demonstrated when He, in turn, handed over the Jews, who were disobedient to Him and sinned, to the Gentiles, not only to be burdened by usury but to be troubled by every sort of shame, a good deal more dreadfully than the Gentiles had been when He gave them over to the Jews. Thus He foretells in this book, chapter twenty-eight, and in this chapter (v. 6) He adds that they will lend to the Gentiles on this one condition, that “they hear the voice of the Lord” (v. 5). This is as though He said: If they do not hear, not only will they not lend on interest, but they will be like, or even more wretched than, the Gentiles, as it also happened.

Today, however, since the Jews have ceased to be the people of God, when the Law is abrogated, and when they through their godlessness and blasphemy have deserved the wrath of God, usury is not to be permitted them; but they are to be controlled by the laws of the people among whom they live. If you closely examine this text, moreover, it does not command them to lend on interest Nor are these words of law, but rather words of promise, when He says: “If only you will obey the voice of the Lord … you shall lend to many nations” (vv. 5–6). But the words of promise, unlike the words of the Law, are not given to man to fulfill. Fulfillment belongs to God alone, who gives them this promise. Therefore the meaning should be like this: “When you hear the voice of the Lord, then through the action of God the Gentiles will be brought into such a plight that they will pay usury even when you neither plan nor seek it; and they will be subject to you in everything, and you will rule over them with all their goods, so that you may seize, demand, and take usury as you please. It is the Lord who will subject the Gentiles to you in this way, and He will humble them through you.”

4. But there will be no poor among you.

A most beautiful order, but one that was never kept. Therefore this law of Moses also remains in words only so far as the whole people is concerned. If begging is forbidden to this people, by what right is it set up among Christians by law, as though it were something sacred? Poverty is extolled, but to the end that it may be relieved. Then spiritual poverty is praised (Matt. 5:3); but the external kind is commanded to be corrected, no differently from other disadvantages of one’s neighbor. It is amazing that those who boast of outward poverty do not also take upon themselves and acknowledge wounds, sicknesses, imprisonments, nakedness, or exile, hunger, thirst, swords, dangers, deaths, sins, the devil, and all other evils by means of new vows set up for these, just as they have done for poverty. Thus one would endure sickness; another, prison; another, hunger; another, sins or the devil. For Christ commands that these things as well as poverty be cared for and improved among our neighbors. “I was weak [He says], and you did not visit Me” (Matt. 25:43).

But instead of sickness and wounds our boasters of poverty carry about a sleek skin and stuffed flesh, worse than the profligates and harlots; instead of places of exile they have homes prouder than the palaces of kings; instead of suffering hunger they consume the storehouses of all; instead of thirst they have full cellars; instead of death they have the most pleasant and secure life. Then they sing to us the glory of enduring poverty, which the Lord has commanded to be abolished, that we might follow the example written for Christian people, in Acts (4:34): “Nor was anyone among them who lacked.” Therefore there ought to be no poverty or begging among the people of God; there should be care and concern to make any poverty and begging unnecessary, that you may know that the mendicant orders and all who display, and boast of, external poverty are disciples and servants of Satan, who rage directly contrary to the Lord and His Christ (Ps. 2:2). In like manner, there ought to be no sickness, hunger, thirst, exile, death, sin, or devil among the people of God; there should be care and concern that if any such thing happens among them, it be removed as quickly as possible and care be taken that it be not found among them. Poverty, I say, is not to be recommended, chosen, or taught; for there is always enough of that by itself, as He says (John 12:8): “The poor you always have with you,” just as you will have all other evils. But constant care should be taken that, since these evils are always in evidence, they are always opposed. You see, therefore, what the institution of the vow of poverty is, and what that whole kingdom of the papacy is.

7. If there is among you a poor man, one of your brethren.

It is the way of the world that when a law has been set up, men soon discover how it can be evaded. Hence Moses here anticipates the fraud which would happen under that law of release in the seventh year, when avarice and human hardness would think, if the year of release happens to be soon: “What shall I do? Shall I give a loan? But the year of release comes after a few months, and I shall have given it in vain, since there will be no hope of getting it back and no right to demand repayment.” Against this Moses inveighs here with amazingly sharp words, calling it a hard heart, a word of Belial, an iniquitous deed, an evil eye, finally even a sin that cries to God. Thus you can see that Moses also agrees with Christ in his instruction about lending; for he commands that the loan be given under the threat of such great sins, even if there is no hope of getting it back and no right to demand repayment. The summary, then, of this teaching is this: the poor should be cared for with love.

12. If your brother is sold to you.

Moses recalls the law of Ex. 21:2 ff and refers it to this law concerning the release of loans, because this also speaks of the poor who, under the compulsion of poverty, sold themselves and received a loan, as it were, since they made themselves debtors with their own bodies. However, he adds at this place that they are not to send him away empty-handed when he is set free. For this he gives two reasons: first, they are to remember that they, too, were slaves in Egypt; secondly, because with him the bondman has been a wage earner in a twofold sense (v. 18). This, I think, is said because the one who is sold to another inflicts double harm on himself: first, he serves another, and everything he gains accrues to his master; secondly, he meanwhile neglects his own business, and what he gains for his master he could have gained for himself. Therefore it would be most reprehensible to send him away altogether empty-handed. Hence he says: “It shall not seem hard to you” (v. 18).

Moses always adds magnificent promises. God has blessed, blesses, and will bless those who do these things, to keep them from doubting that they will be richly compensated if they either give something or lend something as Christ says (Luke 6:38): “Give, and it will be given you.” Hence also that proverb of Solomon (Prov. 19:17): “He who has pity on the poor lends to the Lord”; and again (Prov. 14:31): “He honors the Lord who has pity on the poor,” and many others. On the other hand, the Scriptures are full of threats against those who do not have pity. However, this means nothing to that deaf godlessness and unbelief which imagines that God is either joking or lying with such words, and which deserves to be robbed of the good things both of this life and of the life to come, just as that fool Nabal on Carmel withheld from David, was soon deprived of his life, and kept nothing (1 Sam. 25).