He shall not exchange it or make a substitute for it, good for bad, or bad for good; and if he does in fact substitute one animal for another, then both it and the substitute shall be holy.
This is part of the law concerning singular vows, extraordinary ones, which though God did not expressly insist on, yet, if they were consistent with and conformable to the general precepts, he would be well pleased with. Note, We should not only ask, What must we do, but, What may we do, for the glory and honour of God?
Commenting on Leviticus 27:1-13
And if it be any unclean beast, of which they do not offer a sacrifice unto the Lord,.... Any creature, excepting a dog, the price of which was not to be brought into the house of the Lord; besides oxen, sheep, goats, rams, and lambs; though some understand it even of such that have blemishes on them, and so not fit to be offered unto...
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. (g) That is, consecrate to the Lord.