27And if it is an unclean animal, then he shall buy it back at the valuation, and add a fifth to it; or, if it is not redeemed, it shall be sold at the valuation.
Only clean animals were fit for sacrifice and could not be redeemed. If the firstborn animal was unclean, it could be redeemed.
27 And if it is an unclean animal, then he shall buy it back at the valuation, and add a fifth to it; or, if it is not redeemed, it shall be sold at the valuation.