John Calvin Commentary Leviticus 27:11

"And if it be any unclean beast, of which they do not offer an oblation unto Jehovah, then he shall set the beast before the priest;" — Leviticus 27:11 (ASV)

And if it be any unclean beast. Moses now, in the second place, discusses animals that God commands to be sacrificed to Him if they are suitable, and He does not allow the vow to be altered. But if they are imperfect or unclean, He sets forth the rule for their redemption.

However, the question arises here: How could it be permissible to vow what God had forbidden to be offered to Him, and thus had prohibited from being brought into the temple as unclean? Surely, if it had occurred to anyone to sacrifice an unclean animal, the superstition would be rejected; indeed, expiation would be needed.

But here, in my opinion, another kind of offering is being referred to, one that did not invalidate the sacrifices and service of God by contradicting the commands of His Law. There was therefore nothing strange in His accepting such a vow, though He punishes its levity with a monetary fine.

For instance, suppose a strong and proven horse was in danger; its master made a vow that if it was saved, he would be obligated to pay its price. The same principle applied in other cases. To vow was simply to commit to God’s faithfulness and protection whatever they wished to preserve.

This led to an excessive frequency of vows, which nevertheless had to be fulfilled in some way, so that God’s sacred name would not be exposed to ridicule. God left this valuation to the priest's discretion.

However, if an animal could be offered in sacrifice, no redemption was allowed. If anyone had substituted another animal or paid its price, he was punished for his fraud, because both (that is, the animal and its substitute or price) were consecrated to God.

The valuation imposed on one who had vowed is irreversible, since God simply commands the Israelites to adhere to the judgment of the priest and to abide by the assessment, as it is called, enjoined on them as a fixed rule. In addition, they were to add a fifth part to the price appointed by the priest as an additional fine.