"But the firstborn of an ox, the firstborn of a sheep, or the firstborn of a goat you shall not redeem; they are holy"—Numbers 18:17.
It is forbidden to "redeem" a firstborn animal [i.e., exchange it for another animal or money that would be used to purchase another animal for sacrifice. Rather, it itself must be offered as a sacrifice].
It is, however, permitted to sell [a blemished] firstborn animal.
