John Gill Commentary Deuteronomy 23:20

"unto a foreigner thou mayest lend upon interest; but unto thy brother thou shalt not lend upon interest, that Jehovah thy God may bless thee in all that thou puttest thy hand unto, in the land whither thou goest in to possess it." — Deuteronomy 23:20 (ASV)

Unto a stranger you may lend upon usury
To any Gentile, though some Jewish writers except the Edomites and Ishmaelites, as being brethren, and restrain it to the seven nations of Canaan; but it seems to design one that was not an Israelite, or a proselyte of righteousness, and especially to regard such that traded and merchandised, as the Gentiles very much did, and especially their neighbours the Phoenicians; and of such it was lawful to take interest, as it was but reasonable, when they gained much by the money they lent them, and as it is but reasonable should be the case among Christians in such circumstances; this is to be regarded not as a precept, but as a permission:

but unto your brother you shall not lend upon usury ;
which is repeated, that it might be taken notice of, and carefully observed:

that the Lord your God may bless you in all that you set your
hand to, in the land where you go to possess it ;
for their charity, humanity, and the kind usage of their poor brethren in distress, would not pass unnoticed by the Lord; but he would make the land they tilled fruitful, and their vineyards and oliveyards to produce abundance, and their flocks and their herds to increase greatly, which would be sufficient and more than a recompense for all that they had freely lent unto their brethren, without taking any usury of them.