Bechorot 9 - A Case of Uncertain Firstborn Donkey
If one has a female donkey that had not previously given birth, and it bore two males, he gives one lamb to a kohen as redemption. Although he does not know which one is the firstborn, but one of them is, and it needs redemption. If it bore a male and a female, then it could be that the male is the firstborn, in which case redemption is needed, but it could also be that the female was born first, and then redemption is not required. Accordingly, he separates a sheep as a possible redemption, but keeps it, because the kohen cannot prove that the sheep belongs to him by right. As always in money matters, the burden of proof is on the claimant, who in this case is the kohen.
Before the firstborn donkey is redeemed, it is forbidden for benefit - so says Rabbi Yehudah. What is his reason? - Is there anything that requires redemption that would be permitted to use before redemption? - Certainly not! However, Rabbi Shimon says that it is permitted. What is his reason? - Is there anything whose redemption (sheep) is permitted to use while it itself is forbidden? Of course not!
Art: John Singer Sargent - Donkeys in a Desert