Commentary for Deuteronomy 23:21

לנכרי תשיך UNTO AN ALIEN THOU MAYEST LEND UPON INTEREST (according to Rashi: TO AN ALIEN THOU MAYEST PAY INTEREST) — but not to thy brother. Such a prohibition which is not plainly stated but can only be drawn by inference from a positive command is itself regarded only as a positive command — so that one who pays interest to his brother transgresses two negative commands: לא תשיך in v. 20, ולאחיך לא תשיך in v. 21 and a positive command לנכרי תשיך — ולאחיך לא (cf. Sifrei Devarim 263:1; Bava Metzia 70b; also cf. Rashi on Deuteronomy 14:20).

לנכרי תשיך, you are allowed to pay interest to the gentile (on your loan from him) and you must not betray his trust by citing the prohibition to charge or collect interest.

לנכרי תשיך ולאחיך לא תשיך. "You may pay interest to a Gentile but you must not pay interest to your brother (the Jew)." Why did the Torah repeat here once more that we must not pay interest to a Jew when this had already been stated in verse 20? Our sages in Baba Metzia 75 say that the repetition is to teach that anyone doing this is guilty of the violation of two separate negative commandments. Perhaps the Torah meant here that although the interest you give to your fellow Jew ultimately goes to the Gentile, this is prohibited when the go-between is a Jew and the Jew does not know the Gentile in question either through having borrowed from him or through holding some kind of collateral belonging to the Gentile in question. This is also the halachah.