Shall add a fifth part - This was probably intended to prevent rash vows and covetous redemptions. The priest alone was to value the thing; and to whatever his valuation was, a fifth part must be added by him who wished to redeem the consecrated thing. Thus, if the priest valued it at forty shekels, if the former owner redeemed it he was obliged to give forty-eight.

But if he will at all redeem it,.... The owner of it, or he that has devoted it, if he is determined to have it again at any rate:
then he shall add a fifth part thereof unto thy estimation; he shall give the full price for it, as rated by the priest, and for which it might be sold to another man, and a fifth part of the value of it besides; this was done that the full price might be paid for it, the priest not knowing, as it might be, the worth of it so well as the owner; and that the value of consecrated things might be kept to, and to make men careful how and what they devoted, since, though redeemable, they were obliged to pay a large price for them.

But if the person vowing wanted to redeem it, he was to add a fifth above the valuation price, as a kind of compensation for taking back the animal he had vowed (cf. Leviticus 5:16).