Parashas Behar · Aliyah 2 · Passuk 25:14
Chabad.org — English translation + Rashi
Torah Temimah cites Bava Metzia 51a.
English: “You shall not wrong.” The Rabbis taught: How much must the ona’ah be so that a person may retract? Four silver [ma’ah] from a sela — one twenty-fourth of the purchase.
Classical derasha (Bavli Bava Metzia) — measure of actionable ona’ah.
Monetary ona’ah of one twenty-fourth of the purchase price (or more) gives the wronged party the right to retract the sale.