Bava Kamma 71B

להכי פטרי רבנן; אלא למאן דאמר דרבנן, אמאי פטרי רבנן?

this reason that the Rabbis exempt the thief from the fourfold or fivefold payment when he slaughters the animal on Shabbat, as slaughter on Shabbat does not render the meat permitted for consumption. But according to the one who says that the product of a prohibited act on Shabbat may be eaten by Torah law but is forbidden by rabbinic law, why do the Rabbis exempt the thief when he appoints an agent to slaughter the animal for him and the agent performs this action on Shabbat?

אשארא ⁠– אעבודה זרה ושור הנסקל.

The Talmud answers: When the Rabbis exempt him from the fourfold or fivefold payment, they are not referring to the case where the thief’s agent slaughters the animal on Shabbat. Rather, they were speaking of the rest of the cases listed in the baraita, namely, an agent who slaughters the animal for idol worship and one who slaughters an ox that is sentenced to be stoned.

ורבי מאיר ⁠– אמאי מחייב שוחט לעבודה זרה?

The Talmud asks another question with regard to the interpretation of the baraita presented above: And with regard to R' Meir, granted he maintains that the legal status of an act of slaughter not fit for accomplishing its full ritual purpose is nevertheless considered an act of slaughter. But why is the thief liable to pay the fourfold or fivefold payment when his agent slaughters the animal for idol worship?

כיון דשחט בה פורתא ⁠– אסרה; אידך ⁠– איסורי הנאה הוא, ולא דמריה קא טבח (ולא דידיה קא טבח)!

Once he slaughtered the animal a bit, at the very start of the act of slaughter, he has prohibited the animal, with regard to deriving benefit, as an animal sacrificed to idolatry. When he slaughters the other part it is already prohibited with regard to deriving benefit, which means that it is not an animal that belongs to its owner that he slaughters, and it is not an animal that belongs to him that he slaughters. Since deriving benefit from the animal is prohibited, it has no value; therefore, there is no ownership.