Musar su Deuteronomio 15:3

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.