Thou shalt not lend upon usury to thy brother; usury of money, usury of victuals, usury of any thing that is lent upon usury:

Wesley's Notes

23:19 Thou shalt not lend upon usury to thy brother - To an Israelite. They held their estates immediately from God, who while he distinguished them from all other people, might have ordered, had he pleased, that they should have all things in common. But instead of that, and in token of their joint interest in the good land he had given them, he only appointed them, as there was occasion, to lend to one another without interest. This among them would be little or no loss to the lender, because their land was so divided, their estates so settled, and there was so little a merchandise among them, that it was seldom or never they had occasion to borrow any great sums, but only for the subsistence of their family, or some uncommon emergence. But they might lend to a stranger upon usury, who was supposed to live by trade, and therefore got by what he borrowed: in which case 'tis just, the lender should share in the gain. This usury therefore is not oppressive: for they might not oppress a stranger.

Jamieson-Fausset-Brown Bible Commentary

19, 20. Thou shalt not lend upon usury to thy brother . Unto a stranger thou mayest lend upon usury-The Israelites lived in a simple state of society, and hence they were encouraged to lend to each other in a friendly way without any hope of gain. But the case was different with foreigners, who, engaged in trade and commerce, borrowed to enlarge their capital, and might reasonably be expected to pay interest on their loans. Besides, the distinction was admirably conducive to keeping the Israelites separate from the rest of the world.

Matthew Henry's Concise Commentary

23:15-25 It is honourable to shelter and protect the weak, provided they are not wicked. Proselytes and converts to the truth, should be treated with particular tenderness, that they may have no temptation to return to the world. We cannot honour God with our substance, unless it be honestly and honourably come by. It must not only be considered what we give, but how we got it. Where the borrower gets, or hopes to get, it is just that the lender should share the gain; but to him that borrows for necessary food, pity must be showed. That which is gone out of thy lips, as a solemn and deliberate vow, must not be recalled, but thou shalt keep and perform it punctually and fully. They were allowed to pluck and eat of the corn or grapes that grew by the road side; only they must not carry any away. This law intimated what great plenty of corn and wine they should have in Canaan. It provided for the support of poor travellers, and teaches us to be kind to such, teaches us to be ready to distribute, and not to think every thing lost that is given away. Yet it forbids us to abuse the kindness of friends, or to take advantage of what is allowed. Faithfulness to their engagements should mark the people of God; and they should never encroach upon others.