As, if it enters your mind that it is prohibited by Torah law the Mishnah here would be difficult, for as soon as 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 a non-sacred animal slaughtered in the Temple courtyard. 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.
אמר ליה רב אחא בריה דרבא: כי קא מחייב נמי – אההוא פורתא. אמר ליה רב אשי: לא תידחיה, ״וטבחו״ כולו בעינן – וליכא.
Rav Aḥa, son of Rava, said to Rav Ḥavivi: It is possible to explain the Mishnah even according to the opinion that the halakhic ramifications of slaughtering are in effect throughout the slaughter, and even if one maintains that it is prohibited by Torah law to benefit from a non-sacred animal slaughtered in the Temple. As, when does the thief become obligated to pay the fourfold or fivefold payment as well? It is when he performs that first bit of slaughter in the beginning, before the animal becomes forbidden. Rav Ashi said to Rav Huna: Do not dismiss Rav Ḥavivi’s suggestion with this explanation. The verse: “If a man steal an ox or a sheep, and slaughter it” (Exodus 21:37), indicates that in order to impose liability to pay the fourfold or fivefold payment we require that the thief slaughtered it completely, and after having slaughtered it just a bit there is no complete slaughter yet.
אלא קשיא! אמר ליה, הכי אמר רב גמדא משמיה דרבא: כי קא מחייב – כגון ששחט מקצת סימנין בחוץ, וגמרן בפנים.
Rav Huna said to Rav Ashi: But if you are correct, the Mishnah is difficult. Rav Ashi said to Rav Huna: This is what Rav Gamda said in the name of Rava: When does the Mishnah state that the thief is obligated to pay the fourfold or fivefold payment? It is in a case where the thief slaughtered part of the simanim outside the Temple, and finished slaughtering them inside the Temple. Therefore, the animal became prohibited with regard to deriving benefit only at the final stage of slaughter, concomitant with the liability to pay the fourfold or fivefold payment.