Charles Ellicott Commentary Leviticus 27:21

"but the field, when it goeth out in the jubilee, shall be holy unto Jehovah, as a field devoted; the possession thereof shall be the priest`s." — Leviticus 27:21 (ASV)

But the field, when it goes out in the Jubilee.—That is, when it leaves the possession of the purchaser, since in the Jubilee every buyer was required to part with the land that he bought .

Shall be holy to the Lord, as a field devoted.—It will not revert to the original owner who first vowed it and, after refusing to redeem it, fraudulently sold it. Instead, it becomes God’s property, like all devoted or banned things .

According to the authorities during the Second Temple, however, the import of the law set forth in Leviticus 27:20-21 is as follows:

If the vower of the field does not redeem it before the Jubilee year, and the field is then still in the possession of the Temple treasurer, who has control of all the things consecrated by vow in this way; or if the Temple treasurer has sold the field to another person who possesses it, then the original owner or vower can no longer redeem it.

Instead, in the year of Jubilee, it reverts from either the Temple treasurer or the purchaser to the priests on duty in that year, and they add it to their pasture fields. These priests, however, have to pay the valuation money for it.