Sefer Hachinuch here writes that Ramban holds that when the Torah says you should charge interest to a non-Jew, it means that you can, not that you must. However, we already know that we can charge a non-Jew interest. So what is Ramban's reasoning?
1 Answer
Read on: Ramban says that if someone charges interest of a Jew, they have not just violated the "don't-do" of don't charge a Jew interest, they have also violated the "yes-do" of instead, you may charge the non-Jew. Just a way of strengthening the prohibition.