MISHNA: In the case of one who slaughters an animal on Shabbat or on Yom Kippur, although he is liable to receive the death penalty, his slaughter is valid.

GEMARA: Rav Huna says that Ḥiyya bar Rav taught in the name of Rav: If one slaughtered an animal on Shabbat and Yom Kippur, although the slaughter is valid, consumption of the animal is prohibited for that day, and the members of the company of Sages, i.e., those in the academy, tended to say that this halakha is the opinion of Rabbi Yehuda.

The Gemara asks: Which opinion of Rabbi Yehuda? Rabbi Abba said: It is the opinion of Rabbi Yehuda with regard to preparation for Shabbat, as we learned in a mishna (Shabbat 156b): One may cut the gourds before an animal on Shabbat, provided that they were picked prior to Shabbat. And likewise, one may cut an animal carcass to place before the dogs on Shabbat. Rabbi Yehuda says: If it was not already a carcass prior to Shabbat, it is prohibited to cut it or even move it on Shabbat because it is not prepared for use on Shabbat. Apparently, since it was not prepared from yesterday, it is prohibited. Here too, in the mishna where an animal was slaughtered on Shabbat or Yom Kippur, since it was not prepared from yesterday, it is prohibited.

Abaye said to Rabbi Abba: Are the cases comparable? There, in the mishna in tractate Shabbat, initially the animal is prepared for use by a person, as it was prepared for slaughter, and now that it died without slaughter on Shabbat it is prepared for dogs. But in the mishna here, initially the animal is prepared for use by a person and now after it was slaughtered it remains prepared for use by a person. Rabbi Abba rejects that distinction: Do you hold that an animal during its lifetime is designated for consumption and therefore is prepared for use by a person? On the contrary, an animal during its lifetime is designated for breeding.

Abaye asked: If that is so that an animal is not designated for consumption, according to Rabbi Yehuda, how do we slaughter an animal on a Festival? Rabbi Abba said to Abaye: During its lifetime, the animal is designated for consumption and designated for breeding. If it was slaughtered, it is retroactively clarified that it was designated for consumption; if it was not slaughtered, it is retroactively clarified that it was designated for breeding.

But isn’t it so that Rabbi Yehuda does not accept the principle of retroactive designation? From where do we derive that this is Rabbi Yehuda’s opinion? If we say that we learn it from that which is taught in the following baraita, there is no proof.

It is taught in a baraita (Tosefta, Demai 8:7): In the case of one who purchases wine from among the Samaritans just before Shabbat, and presumably teruma and tithes were not separated, he acts as follows: If there are one hundred log of wine in the barrels, he says: Two log that I will separate in the future are teruma, as the mandated average measure of teruma is one-fiftieth; ten log are first tithe; and a tenth of the remainder, which is nine log, are second tithe. And he deconsecrates the second tithe that he will separate in the future, transferring its sanctity to money, and he may drink the wine immediately, relying on the separation that he will perform later, which will clarify retroactively which log he designated for the tithes and for teruma. This is the statement of Rabbi Meir. Rabbi Yehuda and Rabbi Yosei and Rabbi Shimon prohibit this practice. Apparently, Rabbi Yehuda does not accept the principle of retroactive designation.