Only the firstling of the beasts, which should be the Lord’s firstling, no man shall sanctify it; whether it be ox, or sheep: it is the Lord’s. (Leviticus 27:26)
The first-born of all animals belonged to God, and no one could give to God what was already His. These already belonged to Him (Ex. 13:2, 12; 22:30). An unclean animal, however, might be redeemed. If he did not redeem it the animal was sold.