Or HaChaim on Deuteronomy 23:21/אור החיים על דברים כ״ג:כ״א

1 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.

2There is also an allusion here to a subject debated by the codifiers and later authorities in Yoreh Deyah 159. Some authorities, including Rabbi Joseph Karo, hold that extending loans to pagans is permitted only when the Jew finds it difficult to make a living without doing this even though there is general agreement that the Torah has permitted the practice. The wording לנכרי תשיך may be understood as "you may extend loans with interest to Gentiles, provided that as a result of this permission you will not ultimately charge interest on loans to Jews."