Towards the end of Parashat Bo, the Torah presents several mitzvot that serve to commemorate the Exodus from Egypt, including commands relevant to firstborn people and animals. One such command is that of peter chamor – the obligation to “redeem” a firstborn donkey (13:13). The Torah requires the owner of a firstborn donkey to redeem the animal by giving a sheep to a kohen, and if the owner refuses, then he is required to kill the donkey, as a penalty for refusing to give a sheep.
The Gemara in Masekhet Bekhorot (11a) establishes that if one does not have a sheep, he may redeem his firstborn donkey by paying a kohen the donkey’s value. In the Gemara’s words, “It should be no more stringent than hallowed property, and the Torah said ‘a sheep’ not as a stringency, but as a leniency.” This means that a firstborn donkey cannot possibly be treated more stringently than actual hekdesh – hallowed articles – which can be “redeemed” and divested of their status of sanctity by paying their value to the Temple treasury. When the Torah speaks of using specifically a sheep to redeem a firstborn donkey, the Gemara explains, it establishes a measure of leniency, allowing the owner to redeem the donkey with a sheep even if the donkey’s value considerably exceeds that of the sheep. One is permitted to redeem a donkey for its full price using money or objects of value, but the Torah gives him the option of saving money by using a sheep that is worth far less than the donkey.
The Gemara formulates this halakha by presenting the case of a person who does not have a sheep with which to redeem the donkey. This formulation is adopted also by the Rambam, in his codification of this law (Hilkhot Bikkurim 12:11). The implication, seemingly, is that optimally, the owner should give a kohen a sheep, as the Torah commanded – “tifteh be-seh” (“redeem with a sheep”). It appears, at least at first glance, that although redeeming the donkey with money or objects of value has the effect of divesting the donkey of its status of sanctity – because, after all, “It should be no more stringent than hallowed property” – this is not the preferred way of performing the mitzva. (One might even go so far as to say that one thereby does not fulfill the mitzva at all; he has succeeded in divesting the animal of its hallowed status, but is not credited with the fulfillment of a mitzva, since he did not do what the Torah commands.) Indeed, the Beit Yosef and Perisha (Y.D. 321) infer from the formulation of the Gemara and the Rambam that one should redeem a peter chamor with a sheep, and it is only when this option is not available that other means of redemption are allowed.
However, Rav Moshe Sternbuch (Teshuvot Ve-hanhagot, vol. 4, Y.D. 245) cites one source (Divrei Chamudot) which argues with this conclusion. It is possible that the Gemara and Rambam speak specifically about somebody who does not have a sheep because it is only in such a case that a person would want to use something other than a sheep to redeem the peter chamor. Only a fool would want to pay the full value of the donkey instead of discharging his obligation by using a sheep, and thus it may be for this reason that the Gemara and Rambam formulated the halakha in this fashion, addressing specifically the case of a person who does not own a sheep. According to this view, there is no preference at all to using a sheep over money or other objects of value for the obligation of peter chamor.
Furthermore, in contrast to the Gemara and Rambam’s formulation, the Tur (Y.D. 321) writes, “Just as one may redeem it with a sheep, so may one redeem it with whatever he wishes” – clearly indicating that there is no preference to redeeming a firstborn donkey with a sheep. The Shulchan Arukh (Y.D. 321:5), however, cites the Rambam’s formulation, likely reflecting the view taken by the author in his Beit Yosef, that optimally one should use specifically a sheep for this obligation.
In practice, Rav Sternbuch writes (in the aforementioned responsum) that one should satisfy the stringent view of the Beit Yosef and use a sheep for peter chamor, rather than redeem the firstborn donkey through some other means.