Jamieson, Fausset and Brown on Leviticus 27:16-24

16-24. if a man shall sanctify unto the Lord some aprt of a field of his possession, &c.--In the case of acquired property in land, if not redeemed, it returned to the donor at the Jubilee; whereas the part of a hereditary estate, which had been vowed, did not revert to the owner, but remained attached in perpetuity to the sanctuary. The reason for this remarkable difference was to lay every man under an obligation to redeem the property, or stimulate his nearest kinsman to do it, in order to prevent a patrimonial inheritance going out from any family in Israel.