Bechorot 34: A Secondary Blemish

A new Mishna explains through Rabbi Eliezer that if one creates even a small mum, blemish, in a bechor, it cannot be sacrificed to the Temple but must be given to a kohen who will sacrifice and consume it.  The Sages say that the animal may be slaughtered due to a secondary blemish that might appear.  Rashi teaches that if the person who made the blemish does slaughter the animal, he may not eat the meat.

We learn in the Gemara that there is no biblical prohibition against benefiting from the forbidden act.  This is because in most cases, performing a prohibited act does not forbid the object other that a small number of biblical references.  These other rabbinic halachot (Rabbi Eliezer and the Sages) are meant as punishment.

Tosafot quote the Sifri, which explains Rabbi Eliezer's words differently.  Benefitting from some an animal is biblically prohibited based of Devarim (14:3) where the Torah forbids us to eat anything that is "abominable" which would include this prohibited bechor.