Jamieson, Fausset and Brown on Deuteronomy 15:4

4. Save when there shall be no poor man among you--Apparently a qualifying clause added to limit the application of the foregoing statement [De 15:3]; so that "the brother" to be released pointed to a poor borrower, whereas it is implied that if he were rich, the restoration of the loan might be demanded even during that year. But the words may properly be rendered (as on the Margin) to the end, in order that there may be no poor among you--that is, that none be reduced to inconvenient straits and poverty by unseasonable exaction of debts at a time when there was no labor and no produce, and that all may enjoy comfort and prosperity, which will be the case through the special blessing of God on the land, provided they are obedient.