Parashas Behar · Aliyah 2 · Passuk 25:16
Chabad.org — English translation + Rashi
Torah Temimah cites Sifra (Torat Kohanim).
English: “For it is a number of crops that he is selling you.” This teaches that he is selling him only the crops [i.e., the usufruct until Yovel], not permanent ownership.
Classical derasha (Sifra).
A sale of ancestral land is in essence a sale of the remaining crop-years until Yovel; permanent alienation is not effected.