Who did not redeem the field and who sold the field to somebody else?
Rashi: The owner did not redeem the field and the Gizbar of Hekdesh sold it to a third person.
In Erchin (25b), Tana'im argue about if the Gizbar sold it, whether or not it goes to Kohanim. The one who says that it does, we should infer that if he did not sell it, it does not go to 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, for the son is in place of his father.