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. Rabbenu Tam offers an alternative explanation. He suggests that the Mishnah 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 Mishnah 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.