Somebody slaughters an animal that was a bechor (firstborn animal) and sells its meat. But it was later found out that he never showed the bechor to an expert before slaughtering it. In this type of situation, it is prohibited to benefit from the bechor. Whatever meat the buyers ate, they ate. But he has to return the money they paid him. He has to give them a refund. He lied to them. He gave everything under false pretenses, and therefore it is a mekach ta'ut. The whole deal here is invalid. So he has to return the money to them. Whatever meat wasn’t eaten has to be returned. The meat has to be buried, as is the law for the meat of an unblemished bechor that died. The seller has to return the money to them, since he sold them meat that can’t be used. The sale is a mekach ta'ut. It is a totally invalid sale.
Likewise, if somebody slaughtered a cow and sold it, and later it was discovered that he sold an animal that was a terefah, he knew that it was a terefah. He knew that it was a problematic animal that can’t be eaten, but he sold it anyway. Whatever meat the buyers ate, they ate. But whatever they spent on it, whatever they paid, has to be returned to them. Whatever meat they did not eat, they have to return to the seller, and he has to return the money to them.
When it comes to the meat of a terefah, it’s different from the meat of a bechor. We said that the meat of a bechor that wasn’t eaten has to be buried. That’s because it’s prohibited to benefit from it. Here, the seller can benefit when he gets the meat back. He could either sell it to non-Jews or feed it to his animals.
But if the buyer took the meat that was sold to him as kosher (fit according to Jewish law) meat and sold it to non-Jews, or threw it to his dogs, that was not the seller’s action. The buyer was the one who did that. He could benefit from this meat. He gave it to his animals or to a goy (non-Jews). So the seller didn’t cause the buyer to do something wrong. They have to pay the seller only the price of terefah meat, which is less than kosher meat, and the seller would refund whatever he overcharged them. So if the price of terefah meat, for example, was $2, and he sold it to them for four dollars, he would have to refund them two dollars, whatever the overcharge was, based on the assumption that it was kosher meat, but it really was non-kosher meat. So he has to return to them whatever the difference in price would be.