According to the Mishnah on today’s daf (=page) only the owner of a sacrifice can make it temurah, that is, to attempt to exchange a sanctified animal and transfer the holiness to another, thereby extending the sanctity to both animals. One question that must be discussed is who might be considered the owner of a sacrifice when it is transferred from the Israelite who is obligated in the korban to the kohen who will sacrifice the korban. The Mishnah teaches:
Priests have power to exchange an animal belonging to themselvesand Israelites also have power to exchange an animal belonging to themselves. Priests do not have the power to exchange a sin-offering,a guilt-offeringor a firstling.
From this ruling we see that only the original owner can make a sacrifice temurah, since after the sin-offering or the guilt-offering comes into the hands of the kohen – even though he will partake in eating meat from these sacrifices – it is not considered his for the purpose of temurah.
The difficult case is that of the bekhor – the “firstling.” To whom does a bekhor belong?
The firstborn of a kosher animal – an ox, a sheep or a goat – stands alone among sacrifices in that it is born with intrinsic sanctity, and it is to be given to the kohen for sacrifice in the Temple immediately following its initial development. If, however, it has a blemish that precludes it from being sacrificed, the kohen can slaughter it and eat it anywhere. Even so, it does not lose its sanctity as a firstborn and could not be used for work or sheared for wool – just like any other sacrificial animal. As opposed to other sanctified animals that can be redeemed if they develop such a blemish, a bekhor cannot be redeemed and its kedushah (holiness) cannot be removed.
The continuation of the Mishnah quoted above makes clear that there is a difference of opinion between Rabbi Akiva who believes that a bekhor is similar to the sin-offering and the guilt-offering, inasmuch as it is transferred by the Israelite owner to the kohen, while Rabbi Yohanan ben Nuri argues that it becomes the property of the kohen and therefore should be viewed as belonging to him.