Chulin 10:3-4

Chulin 10:3

If a blemished firstborn animal got mixed up with 100 secular animals, then if a 100 people slaughter all these animals, they are all exempt from giving the portions to a kohein (because his might be the blemished firstborn). If one person slaughters all the animals, only one is exempt. If one slaughters an animal for its owner who is a kohein or a non-Jew, it is exempt from the portions given to a kohein; if he owns the animal in partnership with the kohein or the non-Jew, he must make a sign on the animal (so that onlookers will understand why he isn’t giving the portions from this animal). If a kohein sells an animal to a Yisroel except for these portions, it is exempt from the portions (or their financial equivalent). If a Yisroel told a butcher to sell him the innards of a cow and it included the abomasum (which is one of the portions that is given to a kohein), he must give it to a kohein and he may not deduct it from his butcher’s bill. If he bought from the butcher by weight, he must give the abomasum to a kohein but he may deduct it from his butcher’s bill.

Chulin 10:4

If a convert had a cow that was slaughtered before he converted, he is exempt from giving the portions to a kohein; after he converted, he is obligated. If there is doubt as to whether it was slaughtered before or after he converted, he is exempt because the burden of proof would be on the one who wants to take property from another person, in this case the kohein. The shoulder given to a kohein goes from the knee joint to the shoulder socket of the (right) front leg, the same as with a nazir. The thigh is the corresponding part of the rear leg. Rabbi Yehuda says that the thigh goes from the knee joint to the fleshy part of the leg. The cheek is from the joint of the jaw to the top ring of the trachea.