Leviticus 27:8-12

But if he be poorer than thy estimation, then he shall present himself before the priest, and the priest shall value him; according to his ability that vowed shall the priest value him.

And if it be a beast, whereof men bring an offering unto the LORD, all that any man giveth of such unto the LORD shall be holy.

He shall not alter it, nor change it, a good for a bad, or a bad for a good: and if he shall at all change beast for beast, then it and the exchange thereof shall be holy.

And if it be any unclean beast, of which they do not offer a sacrifice unto the LORD, then he shall present the beast before the priest:

And the priest shall value it, whether it be good or bad: as thou valuest it, who art the priest, so shall it be.