. Question: What the difference between a safek peter chamor (a kosher animal used to redeem a donkey but in doubt if can be used) and a safek bechor (a first born domesticated animal in doubt whether it is a first born)?
A. The Shulchan Aruch (Yoreh Deah 321:2,3) says that you shouldn’t redeem a donkey neither with a calf, or a wild animal or with a slaughtered sheep or a sheep which has a debilitating blemish (Treifa), or with a species illegally mixed (kilayim) or a koy (rather it should be redeemed with a live sheep or goat and then given to the kohen). One can’t redeem with a sheep that looks like a different species but if he did it works. The Rema says that there is an opinion that if he did redeem with it, it is a doubt whether it works.
B. The Be’er Heitiv (2) explains the ramifications of it being a doubt that if the redeemed donkey dies before the redemption creature was given to the kohen the owner is not obligated to give anything to that kohen for the owner can say that maybe the redemption didn’t work and if after it was given to the kohen the redeemed donkey died then the kohen can keep the redemption animal since he is the assumed owner of the animal now. He is the(muchzak) since the owner himself gave it to the kohen.
C. The Shulchan Aruch and Rema in siman 315 says that if a kohen grabbed a safek firstborn sheep or even if the owner gave it to him by mistake he must return it to the owner (and we don’t consider him muchzak).
Answer:By the redemption of the donkey there is at least an obligation to give a proper redemption animal to the kohen but a mistake might have been made and the owner thought this would be a good redemption animal therefore the kohen has a right to keep it. But a sheep where there is a doubt whether it is a firstborn there is a doubt whether there is an obligation at all to begin with.