Leviticus 27
26 Ref But a man may not give by oath to the Lord the first-fruits of cattle which are offered to the Lord: if it is an ox or a sheep it is the Lord's.
27 Ref And if it is an unclean beast, then the owner of it may give money to get it back, in agreement with the value fixed by you, by giving a fifth more; or if it is not taken back, let it be given for money in agreement with your valuing.