Parashas Behar · Aliyah 4 · Passuk 25:25
Chabad.org — English translation + Rashi
Torah Temimah cites Sifra / Kiddushin 21a.
English: “If your brother becomes poor and sells…” This teaches that a person is not permitted to sell his field unless he has become impoverished.
Classical derasha (Sifra / Bavli Kiddushin).
Sale of ancestral land is permitted only under conditions of poverty; one may not sell merely to raise capital while remaining solvent.