Charles Ellicott Commentary Leviticus 27:13

"But if he will indeed redeem it, then he shall add the fifth part thereof unto thy estimation." — Leviticus 27:13 (ASV)

But if he will at all redeem it. —Better, and if he wishes to redeem it, that is, the man himself who vowed it for the sanctuary. The estimate placed on the animal in question was intended for anyone who wished to purchase it, not excluding, however, the person who vowed it.

He shall add a fifth part. —While anyone else could purchase the animal at the valuation placed on it by the priest, its former owner is to pay a fifth more than the valuation price. This was probably intended as a fine for taking back a thing that he promised to the Lord. For the way in which the fifth part was computed during the Second Temple, see Leviticus 5:16.