20 If, however, they do not redeem the field, or if they have sold it to someone else, it can never be redeemed.

Read Leviticus 27:20 Using Other Translations

And if he will not redeem the field, or if he have sold the field to another man, it shall not be redeemed any more.
But if he does not wish to redeem the field, or if he has sold the field to another man, it shall not be redeemed anymore.
But if he does not want to buy it back, and it is sold to someone else, the field can no longer be bought back.