It should be noted that Tosafot point to the Sifri, which offers an alternative view on Rabbi Eliezer’s opinion – that benefitting from such an animal is prohibited by the Torah. According to the Sifri, Rabbi Eliezer derives the prohibition from a passage in Sefer Devarim (14:3), where the Torah prohibits eating “any abominable thing,” which he understands to include meat whose permissibility derives directly from a forbidden act.
As we have learned, a firstborn kosher animal must be given to a kohen and brought as a sacrifice in the Temple. If the animal develops a mum – a blemish that precludes it from being sacrificed – the animal becomes the property of the kohenwho can slaughter it and eat it. The Mishnah on today’s daf (=page) teaches that someone who deliberately causes a mum is not permitted to take advantage of the new status of the animal to make use of it. According to the Mishnah, “if one makes a slit in the ear of a firstborn animal, he must never slaughter it. These are the words of Rabbi Eliezer. Whereas the Sages say: he may slaughter it on account of another blemish, when it appears on it.”
Rashi explains that if the person who made the blemish does, in fact, slaughter the animal he is not permitted to eat its meat.
As is clear from the discussion in the Gemara, there is no biblical prohibition against benefitting from the forbidden act that was done, since generally speaking, performing a forbidden act does not forbid the object, except in a small number of cases specifically enumerated in the Torah. The prohibitions discussed by both the Sages (for a limited time – until the animal suffers another mum) and Rabbi Eliezer (forever) are Rabbinic injunctions established to punish the perpetrator of the prohibition.