Mishnayos Bechoros Perek 5 Mishnah 6

In the case of one who slaughters a firstborn animal and sells its meat, and it was discovered that he did not initially show it to one of the Sages, the halakha is that it was actually prohibited to derive any benefit from the meat. In that case, what the buyers ate, they ate, and the Sages penalized the seller in that he must return the money to them, which they paid for the meat that they ate. And with regard to that which they did not eat, that meat must be buried, and he must return the money that they paid for the meat that they did not eat. And likewise, in the case of one who slaughters a cow and sells it, and it was discovered that it is a tereifa, what the buyers ate, they ate, and what they did not eat, they must return the meat to the seller, who may sell it to a gentile or feed it to the dogs, and he must return the money to the buyers. If the buyers sold it to gentiles or cast it to the dogs, they pay the seller the value of a tereifa, which is less than the value of kosher meat, and the seller refunds the balance to the buyers.