Charles Ellicott Commentary Leviticus 27:18

"But if he sanctify his field after the jubilee, then the priest shall reckon unto him the money according to the years that remain unto the year of jubilee; and an abatement shall be made from thy estimation." — Leviticus 27:18 (ASV)

But if he sanctify his field after the jubile. —If, however, the vow is made after the jubile, the priest is to value the field according to the number of years from the time of the vow to the next jubile year.

And it shall be abated from your estimation. —That is, the years which have elapsed since the last jubile up to the time when he made the vow are to be deducted from the jubile cycle, and hence so many shekels are to be taken off from the original valuation of fifty shekels. Thus, for instance, if he vowed the field at the estimated value of fifty shekels twenty years after the jubile, the priest is only to reckon the thirty years that remain until the next jubile, and is to deduct twenty shekels for the twenty years which have elapsed since the last jubile. Accordingly, the vower would only have to pay thirty shekels, exclusive of the fifth part above the estimated value.