Who did not redeem the field and who sold the field to somebody else?
Rashi and Rashbam: The owner did not redeem the field and the Gizbar of Hekdesh sold it to a third person.
Who is "Ish Acher" coming to preclude?
Erchin, 25b: "Ish Acher" precludes the owner's son - who stands in place of his father 1 - but not his brother.
See Torah Temimah, note 118.
Why does the Torah add the (otherwise superfluous) word Lo Yiga'el Od"?
Erchin, 26a: To teach us that, once the Gizbor sells the field to somebody else, the owner may no longer redeem the field as it was initially - in the form of an inherited field - but he may redeem it in the form of a purchased field - which goes to the Kohanim in the Yovel.
In Erchin (25b), Tana'im argue over, where the Gizbar sold it, whether or not it goes to the Kohanim. According to the one who says that it does, we should extrapolate that, if he did not sell it, it does not go to the Kohanim?
Moshav Zekenim: If the Gizbar sold it to others, it goes to Kohanim after Yovel, but if he sold it to the son, it returns to the original owner, since the son stands in place of his father.
