According to the Mishna, if twin males were born, since one of them is certainly the firstborn, the owner must give a lamb to the kohen; if twins were born – one male and one female – the owner should set aside a lamb, but he may keep it for himself. The reason he can keep it is because his obligation to the kohen is simply financial, and whenever we are faced with a situation of financial doubt, the ruling is ha-motzi me-ḥaveiro alav ha-ra’ayah – the one who wants to claim that his fellow owes him money, must prove his case. Here, since the kohen cannot prove that the firstborn was male, even though he sets aside a lamb in case the firstborn was male, the owner is under no obligation to give that animal to the kohen.
The Mishna continues and teaches that the same lamb can be used to redeem firstborn donkeys many times.
Rashi explains this line by saying that if the kohen chooses to return the lamb to the owner, the owner can give it back to the kohen if he wants to redeem another animal. This needs to be taught in order to emphasize that there is no sanctity involved in this exchange and the lamb remains an ordinary, unsanctified animal. Rabbeinu Tam offers an alternative explanation. He suggests that the Mishna is referring to a case where the lamb remained in the possession of the owner, for example – as we learned – when we are unsure whether the firstborn was male or female and the owner set aside a lamb, but was not obligated to give it to the kohen. The Mishna teaches that the lamb remains the full possession of the owner to the extent that he can use it to redeem another firstborn donkey, or even to act as the animal that was to be exchanged for questionable firstborns many times over.