Leviticus 27:27

Overview - Leviticus 27
He that makes a singular vow must be the Lord's.
The estimation of the person;
of a beast given by vow;
14 of a house;
16 of a field, and the redemption thereof.
28 No devoted thing may be redeemed.
30 The tithe may not be changed.
Treasury of Scripture Knowledge

Leviticus 27:27  (King James Version)
And if it be of an unclean beast, then he shall redeem it according to thine estimation, and shall add a fifth part of it thereto: or if it be not redeemed, then it shall be sold according to thy estimation.

and shall add
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 it.