Parashas Behar · Aliyah 1 · Passuk 25:06
Chabad.org — English translation + Rashi
Torah Temimah cites Bechoros 12b.
English: “For food.” It was taught: With an animal [bought with] Shevi’is one may not redeem a certain firstborn donkey. What is the reason? The Merciful One said “for food” and not for commerce.
Classical derasha (Bavli Bechoros).
An animal purchased with Shemittah produce may not be used to redeem a firstborn donkey, because the verse restricts use to eating, not commerce.