“Examining A Bechor Is Not The Same As Examining A Tereifa”
The commentators ask why examining a firstborn animal to see whether it has a blemish, and thus determining its status, is different from examining the organs of a slaughtered bird or animal to determine whether it is a tereifah [having a major defect which renders it forbidden for consumption].
Rashi (s.v. “Alma…” ibid.) explains that a ruling regarding a kashrut question is not comparable to mesakken [performing a significant repair] because the decision only clarifies and confirms the halacha; the ruling does not establish a new status, nor does it revise the meat’s previous status. However, in the case of a bechor – a firstling [it appears as though] it is the expert’s pronouncement that actually renders the bechor permitted [to be given to the kohen], as evidenced by the fact that R. Meir forbids [utilizing] a firstborn animal that was slaughtered prior to the expert’s pronouncement.
What Is Forbidden?
The Taz (Orach Chayyim 498:9) draws a different distinction. In the case of the questionable tereifa, the rabbi’s ruling is not so significant because he does not change the animal’s chazaka – established status since an animal is presumed to have been born without a defect. Therefore, his ruling is not nir’eh ke’mesakken. In contrast, for the bechor which is also considered to have been born without a defect, the established presumption indicates that it is forbidden for slaughter [outside the Beis Hamikdash] until it is proven that it has developed a valid blemish. Such a ruling has particular significance since it alters the status of the animal and it therefore appears as misakken (a significant repair).
An Established Status
According to the Taz there are certain halachic rulings that cannot be made on Yom Tov. Suppose that before Yom Tov one had a mixture of kosher and non-kosher meat that was forbidden to him because most of that mixture consisted of non-kosher meat. On Yom Tov additional pieces of kosher meat fell into the mixture, and a rabbi is asked to determine whether the mixture is now permitted because it might consist mostly of kosher meat. Since the mixture was initially forbidden, the Taz would not allow the rabbi to issue a ruling on the Sabbath or on Yom Tov to permit the mixture, for such a ruling alters the established status of the mixture.
What We Have Seen
Korban Nesanel (os 40) disagrees, arguing that he has seen numerous rabbis issuing such rulings on Yom Tov. Moreover, he cites the Terumas HaDeshen (siman 54, as cited by the Magen Avraham, Orach Chayyim 323:sk 14), who explicitly permits such rulings.
A Bechor Is Different
Korban Nesanel explains that an expert’s ruling on a bechor’s blemish is akin to a judgment in a monetary lawsuit because it results in a transfer of ownership. The firstborn animal is initially in the possession of the hekdesh, and through the expert’s ruling it is transferred to the possession of the kohen. This type of ruling is similar to a monetary judgment where, as a result of the judge’s ruling, the defendant pays money to the plaintiff [and the Mishna (36b) says that monetary rulings may not be made on the Sabbath or on Yom Tov].
Even though the Terumas HaDeshen permits a rabbi to rule on whether bittul [nullification] has occurred in a mixture, he says that directly causing bittul by intentionally adding kosher meat to a mixture is forbidden because that is certainly considered an act of significant repair [mesakken].