We are studying the sixth and final Mishnah in the fifth chapter of Tractate Bechoros, continuing with the topic we have been discussing: the obligation for an expert to rule that a bechor has a permanent blemish before it is slaughtered. What is the halachah when a person ignores this, slaughters the bechor, and sells it to someone else? This question brings us to the law of mekach taus (a mistaken transaction).
The law of a mistaken transaction:
The rule is that a person who bought an item while unaware of a material fact - whether a matter of halachah or a physical reality - which, had he known it, would have prevented him from making the purchase, may claim that the sale was made under false pretenses and is not binding. The buyer forces the seller to take back the merchandise and refund his money in full.
It is worth noting that there is no middle ground here: the buyer cannot force the seller to provide a partial refund to "even things out" and correct the flaw. Obviously, the parties may reach an agreement between themselves, but according to the strict law, the transaction is entirely canceled - the buyer returns the item, and in return, the seller refunds the money.
"Hashochet es habechor u'mecharo" - One who slaughters a bechor and sells it:
The Mishnah discusses a kohen who slaughtered his bechor and sold it to someone else, "venoda" - and it became known to the buyer after the fact - "shelo her'ahu" - that the kohen did not show the bechor to an expert prior to the slaughter. The bechor is forbidden, and we learned that this is not merely a prohibition against eating it, but that rabbinically, deriving any benefit from it is entirely forbidden and it requires burial. Even if it is a three-legged animal, where it is obvious to everyone that it would be ruled as having a permanent blemish, as long as the official ruling was not given by the expert or by the three men in the synagogue, the slaughter is invalid.
"Mah she'achlu achlu" - Whatever meat the buyer ate in the meantime, he ate. "Veyachazir lahem es hadamim" - Even though the buyer cannot return what he has already eaten, and usually there is no monetary refund except against the return of the item, here the seller is obligated to provide a full refund even though he receives nothing in return. This is a rabbinic penalty for acting improperly and selling the meat of a bechor without bringing an expert to permit its slaughter.
"Umah shelo achlu habasar yikaveir" - The portion of the meat that was not eaten is not returned to the seller in the manner of a regular mistaken transaction, rather the buyer buries it in the ground, for it is forbidden to derive benefit from the meat and he is not permitted to use it. Nevertheless, the seller must also refund the value of this portion. Thus, the kohen returns the full amount of money and receives nothing in return: what was eaten - is eaten, and what was not eaten - is buried.
Selling treifah meat:
From here the Mishnah transitions to a similar topic, and we are no longer discussing a bechor but rather ordinary meat (chullin). An animal that has a critical, permanent, and life-threatening defect that will eventually cause its death is a treifah and is not kosher. There is a presumption that the animals found in a barn are not treifos but are kosher, and therefore, on a Torah level, a person is permitted to slaughter and eat them based on this assumption. However, rabbinically, there is an obligation to check at least the lungs, to ensure they do not have an adhesion (sircha) that makes them a treifah, and one who refrains from doing so eats meat that is not kosher rabbinically, and there is a punishment for this.
The Mishnah discusses a person - not necessarily a kohen, but a butcher or a farmer - who slaughtered an animal and sold its meat, and afterward it became clear that it was not kosher. The case of the Mishnah, simply understood, is that the seller knew very well that the animal was not kosher and sold it anyway: either he knew it was not checked, or he knew it was checked and found to be a treifah. Since he acted dishonestly, he is penalized.
The meat that was eaten: The buyer cannot return it, and on a Torah level, there is no basis for a claim of a mistaken transaction regarding this portion. Nevertheless, the seller is penalized rabbinically and must refund the buyer the money he spent on it, even though he has already benefited from eating it.
The meat that was not eaten: The buyer returns it to the seller, for the transaction is void. Unlike a bechor, where the meat is strictly forbidden for any benefit, with a regular treifah animal, it is permitted to derive benefit from the meat: one may feed it to the dogs and use the hides to make furniture. Therefore, the buyer returns the meat, "veyachazir lahem es hadamim" - and the seller refunds his money due to the mistaken transaction.
The bottom line: The buyer receives a full monetary refund, and the seller only receives back the remaining meat. According to the strict rules of a mistaken transaction, this would not be the halachah, as there is no refund without returning the item. However, here the seller is penalized for acting intentionally.
How can the meat be returned to him, without the concern that he will go and sell it again?
The answer lies in a separate halachah that is not part of our Mishnah: This seller becomes a suspect, and since it is known that he sells treif meat - a public announcement is made about him, and it is forbidden to buy meat from him anymore.
"Mecharo L'nochrim O Hetilo L'klavim":
The Mishnah continues and discusses a case where the uneaten meat is no longer in the buyer's possession: either he sold it to non-Jews, since it is not kosher, or he threw it to his dogs. In this case, "yeshalmu demei hatereifah" - they pay the value of the treif meat. Since the buyer did not suffer a total loss, as he derived financial value from the meat, whether by selling it to non-Jews for a partial payment (since treif meat is worth less than kosher meat) or by feeding it to his dogs, which saves him the cost of their food - the penalized seller only compensates him for his actual loss.
For example: If the meat he did not eat was worth one hundred, and he sold it to a non-Jew for fifty, he receives the remaining fifty from the seller. Fifty plus fifty - one hundred, and it turns out he loses nothing.
In summary: In this Mishnah we learned two cases of a mistaken transaction involving meat. In the case of a bechor that was slaughtered without the permission of an expert - what was eaten they have eaten, what remains is buried because it is forbidden to derive benefit from it, and the seller refunds the full purchase price and receives nothing back. In the case of an animal that was found to be a tereifah - the meat that was eaten is not returned but its money is refunded, and the remaining meat is returned to the seller since it is permitted to derive benefit from it, and if it was already sold to non-Jews or thrown to dogs, the seller pays only the value of the tereifah. In both cases, the basis for the obligation is one: a Rabbinic penalty on the seller who acted improperly. With this, the fifth chapter of Tractate Bechoros is concluded.