רמב"ן — את הנכרי תגוש

Q — The Sifri reads "את הנכרי תגוש" and "לנכרי תשיך" as mitzvot aseh, and Rambam counts them in ספר המצוות (ק"מב, קצ"ח) — a positive obligation to press a גוי for repayment and to lend to a גוי with ribbit. The verses look like they say exactly that. Why does Ramban reject this?

A — Famous Ramban-Rambam machloket. Ramban: "את הנכרי תגוש" is a mitzvat aseh ABOUT YOUR BROTHER, not the נכרי. The verse's main point is "ולא אחיך"; phrasing it positively creates a לאו הבא מכלל עשה — the prohibition-derived-from-positive-statement itself counts as an aseh. So one who pressures a brother for repayment violates both לא תעשה and עשה — but there's no mitzvah to actually press the נכרי. Same exact analysis for ribbit: "לנכרי תשיך" creates an aseh-prohibition against taking interest from a Jew, not a mitzvah to charge interest to a non-Jew. Ramban: "ודבר ברור הוא." (Raavad's השגה on הלכות מלוה ולוה ה,א sides with Ramban.)