Gemara

75b (Mishnah - R. Meir): If a woman had Mumim in her father's house, the father must prove (to collect a Kesuvah) that they came after Kidushin, and the husband's field (wife) was ruined. If she entered the husband's Reshus and Mumim were found, the husband must prove (to be exempt from paying a Kesuvah) that they came before Kidushin, and the Kidushin was invalid.

Question: In the Reisha, the father must bring proof. This is like R. Yehoshua, who says that a woman is not believed (to say that she was a Besulah at the time of Kidushin, and was raped later - 12b). In the Seifa, the must bring proof. This is like R. Gamliel, who says (12b) that she is believed!

Answer (Rava): One Tana taught the entire Mishnah. In the Reisha the Mumim were found when she was with her father, so we assume that they were there from the beginning. In the Seifa the Mumim were found when she was with her husband, so we assume they came after Nisu'in.

76a (Rav Yehudah citing Shmuel): Reuven traded his cow for Shimon's donkey. Shimon was Moshech (brought to his premises) the cow to acquire it. When Reuven went to take the donkey, he found that it was dead. Shimon must prove that his donkey was alive when he did Meshichah (if not, the trade is void and he must return the cow). The Mishnah of a Kalah supports this.

(Rav Nachman bar Yitzchak): He refers to a Kalah in her father's house, regarding the Kidushin money. The father must bring a proof to keep it.

Question (Beraisa): If there is a scab on a wound in the stomach due to a needle, the animal was Tereifah three days before the Shechitah. If not, the one who wants to take money must bring a proof.

If the butcher already paid for the animal, he must bring proof to recover his money. According to Shmuel, the seller should need to bring a proof to keep his money!

Answer: The case is, the butcher did not yet pay. (The seller must bring proof to force the butcher to pay).

Question: Why does the Tana assume that this is the case (without specifying? This is left difficult.)

(Rami bar Yechezkeil): Rav Yehudah misquoted Shmuel. Rather, (when animals are traded) Shmuel puts the burden of proof is on the one in whose Reshus the doubt arose. The (entire) Mishnah of a Kalah supports this (like Rava explained).

Question (Beraisa): If the butcher did not pay yet, the seller must bring proof. According to Shmuel, the butcher should need to bring a proof to avoid paying!

Answer: The case is, the butcher paid. The Tana did not need to specify, for people do not (normally) give their animals before being paid.

Rishonim

Rif: Shmuel taught that if Reuven traded his cow for Shimon's donkey, and Shimon did Meshichah on the cow, and Reuven found the donkey dead before he did Meshichah, Shimon must prove that his donkey was alive when he did Meshichah. The burden of proof is on the one in whose Reshus the doubt arose. The Mishnah of a Kalah supports this. We ask from the Beraisa of a needle. The one who wants to take money must bring a proof. If the butcher did not pay yet, the seller must bring a proof to get paid. According to Shmuel, the butcher should need to bring a proof, for the doubt arose in his Reshus! We answer that the butcher already paid. Sellers do not give the animal before getting the money. Some say that the Halachah does not follow Shmuel. Even though we answered the question, it is a poor answer and we do not rely on it. Rather, no matter where the doubt arose, the burden of proof is on the one in who seeks to take money. Some say that since the Mishnah of Kalah supports Shmuel, this shows that the answer for Shmuel is proper. If not, there is a contradiction about the Halachah! Therefore, the Halachah follows Shmuel.

Rosh (7:16): Shmuel taught that if Shimon did Meshichah, and Reuven found the donkey dead before he did Meshichah, Shimon must prove that the donkey was alive. The Mishnah of a Kalah supports this. We ask from the Beraisa of a needle. The one who wants to take money must bring a proof. If the butcher already paid, he must bring a proof to get back his money. According to Shmuel, the seller should need to bring a proof! We answer that the butcher did not pay yet. It is difficult why the Tana assumes this. Rather, Rami bar Yechezkeil says that Shmuel taught that the burden of proof is on the one in whose Reshus the doubt arose. Rashi says that this refers to Reuven, for we did not find that the donkey died until after (Reuven acquired it through Meshichah) of his cow. The entire Mishnah of a Kalah supports this, like Rava explained. The one in whose Reshus the blemish was found must bring proof.

Rosh (ibid.): The Rif explains that even according to Rami, Shimon must bring proof. Since the donkey was in his premises, we say that the doubt arose in his Reshus, and we assume that it died when he owned it. Had the donkey been in a Simta (a shoulder of Reshus ha'Rabim, i.e. neutral property), Reuven would need to bring proof. This is difficult. The moment Shimon did Meshichah, Reuven acquired the donkey, wherever it is. The Chazakos favor Shimon, both regarding money (he holds the cow) and Guf (the donkey was initially alive when he had it, and it was found dead by Reuven). Why should Shimon need to bring proof? We assume that 'what was found here, was initially here' only regarding an Arusah in her father's house. An animal immediately transferred through Meshichah, even if it is still in the premises of its original owner. Even if the Arusah went to the Arus' house, her father must bring proof until Chupah. Even if she is still in her father's house, her husband must bring proof after Chupah. This shows that we are not concerned for who owns the premises, rather, for who owns the property.

Question (Korban Nesan'el 4): Perhaps the Rif holds that we are concerned for who owns the premises!

Rosh (ibid.): Also, why did Rami tell people not to heed Rav Yehudah? Perhaps Shmuel discusses the normal case, when the donkey is in the original owner's Reshus until Meshichah, but he would agree if it was in a Simta! Surely, Rami holds the opposite of Rav Yehudah. We must say that there is an error in our text of the Rif. It should say 'the owner of the cow must bring proof.

Ran (DH u'Moshach, explaining the Rif according to Ba'al ha'Ma'or): Rav Yehudah teaches that the owner of the property about which the Safek arose must bring proof. This refers to Shimon, who originally owned the donkey. We are unsure whether or not it changed hands before it died. A Safek does not uproot a Vadai. We reject this due to the Beraisa of a needle. Rather, Rami says that the owner of the premises in which the Safek arose must bring proof. We assume that the problem came there. The Rif brings the first version, even though it was rejected, because when the donkey died in Shimon's premises, all agree that Shimon must bring proof. They argue only about the reason.

Drishah (1 DH v'Divrei): The Rif brings Rami's version! Rather, Rami said only 'the owner of the premises in which the Safek arose must bring proof.' Why did the Rif cite Rav Yehudah (that Shimon must bring proof)? The Ran answers, it is because Rami agrees to this when it died in Shimon's house.

Rambam (Hilchos Mechirah 2:14): The one in whose Reshus the Safek arose must bring a proof. If Reuven traded his cow for Shimon's donkey, and Shimon did Meshichah on the cow, and Reuven found the donkey dead, Shimon must prove that his donkey was alive when he did Meshichah. The same applies to all similar cases.

Poskim

Shulchan Aruch (CM 224:1): The one in whose Reshus the Safek arose must bring a proof. If Reuven traded his cow for Shimon's donkey, Shimon was Moshech the cow, and Reuven found the donkey dead, Shimon must prove that it was alive when he did Meshichah. The same applies to all similar cases.

Beis Yosef (DH v'Zeh): The Rif and Rambam seem to rule like Shmuel, that Shimon must bring proof. Some say that if Shimon says that he knows that it was alive when he did Meshichah, or if it was in the swamp, Reuven must bring proof. Since the Rif and Rambam could agree with the Rosh when it was in a Simta, we rule like the Rosh in this case.

Gra (1): The Gemara in Bava Metzi'a 100a supports the Rif. (If we are unsure whether a cow gave birth before or after Levi acquired it through Chalipin, the Gemara said that the one in whose premises the baby lies is Muchzak.)

Rema: Some say that since Reuven acquired the donkey (through Shimon's Meshichah), wherever it is, it is in his Reshus, so he must bring a proof.

Gra (2): This is like Rashi and Tosfos (76b DH Kol), who say that Rami (totally) argues with Rav Yehudah.

SMA (3): (Tosfos says that we must distinguish between our case and a cow that gave birth.) It seems that here we know that the donkey was sold, whereas there perhaps the calf was not sold at all.