Jamieson, Fausset and Brown on Leviticus 27:9-13

9-13. if it be a beast, whereof men bring an offering unto the Lord--a clean beast. After it had been vowed, it could neither be employed in common purposes nor exchanged for an equivalent--it must be sacrificed--or if, through some discovered blemish, it was unsuitable for the altar, it might be sold, and the money applied for the sacred service. If an unclean beast--such as an ass or camel, for instance, had been vowed, it was to be appropriated to the use of the priest at the estimated value, or it might be redeemed by the person vowing on payment of that value, and the additional fine of a fifth more.