Chullin, Chapter 10, Mishnah 3. This chapter is devoted to the matanot, the three gifts of the slaughtered animal (the foreleg, the jaws and the stomach) that belong to the kohen. The previous mishnah taught that a blemished bechor is not subject to this obligation: even though it may be eaten by its owner, its gifts are not handed over to the kohen. Our mishnah asks what happens when such a bechor can no longer be identified, because it wandered into a group of ordinary animals whose gifts are owed to the kohen. It then continues with a series of additional cases in which a person is released from the obligation of the matanot.
The bechor that got lost in the crowd
"Bechor shenit'arev beme'ah": a blemished bechor, exempt from the gifts, became mixed together with one hundred ordinary animals, whose gifts must be given. There is no way to point to the bechor and say, this is the one. The mishnah distinguishes between two situations.
"Bizman sheme'ah shochatin et kulan, potrin et kulan." If the animals belong to one hundred different owners, and each owner slaughters his own animal, then all of them are exempt. Every single owner can turn to the kohen and say, perhaps the animal I slaughtered is the bechor, and from a bechor no gifts are given. Since no claim can be established against any particular person, none of them is required to hand anything over.
"Echad shochet et kulan, potrin lo echad." But if a single person owns the entire group and slaughters all of them himself, we exempt him from the gifts of one animal only. He must give the matanot of all the rest. The reasoning is straightforward: only one of the animals in his possession is a bechor, and all the others are ordinary animals whose gifts are owed. So he gives for every animal except one, and that one is treated as the bechor for which nothing is due.
Animals that are not subject to the gifts
"Hashochet lekohen ulenochri, patur min hamatanot." If a person slaughters an animal belonging to a kohen or belonging to a nochri, an idol worshiper, there is no obligation of matanot at all. The laws of the gifts apply only to the animal of a Yisrael, not to an animal owned by a kohen and not to an animal owned by a non-Jew.
"Vehamishtatef imahen": when a Yisrael brings a kohen or a nochri into the ownership of the animal as his partner, he too is freed from handing over the gifts, because his partner holds a portion in each of the three parts. Still, the mishnah imposes a condition: "tzarich sheyirshom," a mark must be made on the gifts showing that a kohen or an idol worshiper has a stake in them. The concern is how this looks to others. People who see a Yisrael with an interest in the animal are likely to conclude that the beast is entirely his, and that in retaining the foreleg, the jaws and the stomach he is transgressing the Torah. Marking them puts the partnership on display and dispels the suspicion.
A kohen who sells the animal but keeps the gifts
"Ve'im amar chutz min hamatanot, patur min hamatanot." Here the animal came into a Yisrael's hands through a purchase from a kohen, and as the transaction was made the seller specified that the matanot were excluded from the sale and stayed in his own possession. The buyer then bears no obligation of gifts at all. One might have reasoned that the Yisrael, having acquired the animal in its entirety, now owes the kohen the value of the three parts in money. The mishnah rules differently. Those parts were never transferred to the purchaser; the kohen held on to them from the very beginning of the deal. What was never his to hand over cannot generate an obligation upon him.
Buying the innards from a butcher
Someone approaches a butcher and asks to buy the intestines of a certain cow. The mishnah's words are "mechor li bnei me'eha," and it notes that "hayu bahen matanot," among what he purchased was the keivah, the stomach, which is the property of the kohen. The halachah is "notnan lekohen," the buyer hands it over, and the mishnah adds that no reduction is made in the money he paid. Why should the butcher return nothing? Because the purchaser was fully aware of what he was acquiring: he knew the keivah was included in the sale, and he knew it would ultimately have to reach a kohen. That awareness was built into the price he accepted, so he cannot come back afterward with a complaint.
The second case is a sale transacted on the scale: "lakach hemenu bemishkal." The purchaser paid according to weight, and the keivah was counted in that weight. Now the ruling shifts. He still hands the keivah over as required, but this time "menakeh lo min hadamim," its value is subtracted from what he owes. The logic follows the form of the sale: his money bought a certain quantity of flesh, and he paid for every bit of that quantity, the stomach among it. He is therefore entitled to approach the butcher and claim back the price of the piece he was obligated to surrender.