Musar zu Dewarim 23:21

The סמ"ג writes in the introduction to his book that Maimonides views the commandment of charging interest on loans to Gentiles as one of the 613 commandments (Deut. 23,21). He also considers Deut. 15,3 (calling on Jews to insist on repayment of oustanding loans from gentiles) as a positive commandment, in spite of the need to relinquish loans outstanding from Jews in the שמטה year. We do indeed find the ספרי on either of these two verses counting these statements as positive commandments, whereas we do not find this mentioned in the Talmud. In fact, the Talmud Baba Metzia 70 suggests that transactions with Gentiles involving interest are permissible but not mandatory. Charging interest to a Jew is considered a לאו הבא מכלל עשה, a negative commandment resulting from a positive commandment (the commandment to charge interest to gentiles). This is what the סמ"ג writes. I cannot understand how such a statement could be used to refute Maimonides who considers charging a Gentile interest a positive commandment! It seems to me that the quote from the Talmud supports the view of Maimonides.

To quote the Talmud verbatim: "We read in Proverbs 28,8 that 'he who increases his wealth through interest and usury only gathers in for him who is gracious to the needy.' Who is meant by 'the one who is gracious to the needy?' Rav says it is someone like King Shevor [since the money eventually will wind up in the king's treasury]. Rabbi Nachman says that he was told by Rav Hunna that it applies to someone who charged interest to a Gentile [for he will ultimately not benefit thereby]. This statement is challenged by Rava who questions the meaning of the word לנכרי תשיך, suggesting that it means: "you must charge interest,” only to be answered that the meaning is" "you must pay interest." Thereupon the Talmud queries whether it is conceivable that the Torah wishes to encourage us to take loans from Gentiles and to pay them interest?! The Talmud answers that the verse tells us not to pay interest to a fellow Jew. The Talmud counters that we already have an explicit statement governing this eventuality in Deut. 23,21: "You must not charge interest to your brother!" The Talmud answers that the reason the Torah writes two verses on the same subject is to make us culpable for violating both a positive and a negative commandment when charging interest to a fellow Jew.