Bechoros 1:4-5

Bechoros 1:4

If one donkey gave birth before and the other one hadn’t and they gave birth to two males, the owner must give one lamb to the kohein (because one of them is invariably a firstborn). If they give birth to a male and a female, the owner puts one lamb aside for himself. This is based on Exodus 34:20, “You shall redeem the firstborn of a donkey with a lamb.” The redeeming animal can come from sheep or goats, be male or female, large or small, unblemished or blemished, and one can redeem several doubtful donkeys with it. The lamb used to redeem the doubtful donkey goes back into the pen to be counted for animal tithes. If a lamb separated to redeem a donkey dies before being given to a kohein, one may derive benefit from it. [The Gemara clarifies that it’s the kohein who may derive benefit from it because it becomes his property as of the time it’s put aside.]

Bechoros 1:5

One may not redeem a donkey using a calf, a kosher wild animal, a slaughtered animal, a treifa, a crossbred animal, or a koy (an animal that is unclear whether it is wild or domesticated). Rabbi Elazar permits one to redeem with the crossbred offspring of a goat and a sheep because the offspring is certainly a lamb but he prohibits using a koy because its status is in doubt. If the owner gave the firstborn donkey to the kohein (instead of redeeming it), the kohein may not keep it unless he separates a lamb to redeem it.