Gemara

(Beraisa): If Leah checked a garment and lent it to Rachel, and Rachel found a blood stain, she attributes it to Leah. (She assumes that Leah did not check well.)

(Rav Sheshes): This refers to the toil of laundering the stain (Rachel is exempt). However, Leah is Tehorah, and Rachel is Teme'ah.

Kesuvos 75a (Mishnah - R. Meir): If a Kalah had Mumim in her father's house, her father must prove that they came after Kidushin, and the husband's 'field' (wife) became blemished. If she entered the husband's Reshus and Mumim were found, her husband must prove that they came before Kidushin, and the Kidushin was invalid.

(Rava): In the Reisha the Mumim were found in her father's Reshus, so we assume that they were there from the beginning. In the Seifa the Mumim were found in her husband's Reshus, 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. Reuven went to take the donkey, and found that it is 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.) Our Mishnah supports this.

(Rav Nachman bar Yitzchak): He refers to the Reisha (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 (the buyer) 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.

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 on the one in whose Reshus the doubt arose. Our (entire) Mishnah supports this (like Rava explained).

Question (Beraisa): If there is no scab, ha'Motzi mi'Chavero...

If the butcher did not pay yet, the seller must bring proof. According to Shmuel, the butcher must bring a proof to avoid paying!

Answer: The case is, the butcher paid. The Tana did not need to specify, for normally one does not give his animal before he is paid.

Bava Metzi'a 100a (Mishnah): If Reuven traded a cow for Shimon's donkey, and the cow gave birth (elsewhere), and they argue about whether the birth was before or after the sale, they divide the offspring.

Question: The one in whose premises the baby lies is Muchzak. The other must bring proof to take from him!

Answer: The Mishnah is Sumchus, who says that when we are in doubt, we divide the money without an oath.

Bava Metzi'a 42b: An overseer of orphans bought an ox for them. He gave it to a cattle herder (Levi). It did not have teeth. It could not eat, so it died. The orphans found the seller and he returned their money. He is upset that they did not tell him (immediately that it has no teeth, and return it).

The case is, the seller buys and sells animals. He did not know that it had no teeth.

(Rami bar Chama):: The seller swears that he did not know that it had no teeth, and the herdsman pays (the seller) for the meat at a discount.

Rishonim

Rif (Kesuvos 35b DH Gemara): According to Shmuel, if the buyer did not yet pay, the seller must bring proof. Some rule unlike Shmuel, for the Gemara challenged him. We do not rely on the weak answer given. Rather, no matter where the Safek arose, ha'Motzi mi'Chavero Alav ha'Re'ayah. Some say that since our Mishnah supports Shmuel, the answer is a proper answer. If not, there is a contradiction about the Halachah! Therefore the Halachah follows Shmuel.

Rambam (Hilchos Mechirah 20:15): If a needle went all the way through the stomach, if there is no scab, it is a Safek. The buyer must bring a proof, for the Safek arose in his Reshus. If he does not bring a proof, he must pay the seller.

Rosh (Chulin 3:34): If there is no scab, it is a Safek whether it was punctured before or after the sale. Ha'Motzi mi'Chavero Alav ha'Re'ayah, i.e. the buyer must bring a proof. This is even if he did not yet pay for it. The problem was found here (in the buyer's Reshus), so we assume that it arose here, like it says in Kesuvos. The buyer is called ha'Motzi, for presumably the seller did not give the item before receiving the money.

Tosfos (Kesuvos 76b DH Kol): Rami establishes the Mishnah like Rava. The one in whose Reshus the Safek arose must bring a proof, even if he is Muchzak. This is clear from the question about the needle. The buyer must bring a proof even though he did not pay yet. What was the Gemara's question about a cow sold through Chalipin, and we found that it gave birth? It connotes that if it gave birth in the Reshus of the owner of the donkey, even though the Safek arose there, the owner of the cow must bring a proof! We must distinguish between a Safek that arose in one's favor, like there, to one that arose to his detriment, like here (a blemish in the Kalah).

Tosfos (Nidah 58a DH ul'Inyan): Why is Leah exempt from laundering it? In Kesuvos, we say that the one in whose Reshus the Safek arose must bring a proof! I answer that here, Leah merely borrowed it, so it is not considered her Reshus. Also, there we say that if one wants to take money, he must bring proof. Here, Leah is not obligated to launder it.

Maharashdam (CM 309, 449): Many hold like the Rif, that if the buyer did not pay yet, the owner must bring proof. The Magid Mishneh says that the Rashba says that if he did not pay, we take from him without a proof. Tosfos asked from Bava Metzi'a, and answered that if a Safek arose in one's Reshus in his favor, the other party must bring proof. The second answer of Tosfos in Nidah agrees. Regarding a Kalah (the husband must bring proof) because he wants to divorce her, and she has Chazakah of her body, and perhaps he already knew and accepted the blemish, for there is a Chazakah that one does not marry before checking (through his female relatives, who see her at the bathhouse). Tosfos holds that a Muchzak in whose Reshus the Safek arose must bring a proof only when there is another Chazakah, e.g. that a Kalah has no blemish or a cow was alive. Therefore, we say that it arose only now. The Ge'onim apply 'here it was found. Here it arose' even to obligate payment. In a Teshuvah (brought in Tur CM 60), the Rif says that if one put a lien on all his property in a document, and now claims that he did not own property at the time (so it was invalid), and the witnesses do not know, the claimant must bring proof.

Poskim

Shulchan Aruch (CM 232:11): If the Beis ha'Kosos was punctured and a scab formed, if he bought the animal within the last three days, or he bought it more than three days ago but there is no scab, it is a Safek whether it was punctured within three days. Reuven must bring a proof. If he does not, he loses, and he must pay if he did not pay yet.

Beis Yosef (DH ha'Mocher): Rashi explains that if there is no scab, it is a Safek whether it was punctured before or after the sale. The butcher (i.e. the buyer) must bring a proof, for he is the Motzi. Presumably the seller did not give the animal before receiving the money. We do not force one to pay due to Safek. Within three days, Reuven must bring a proof. If he does not, he loses, and he must pay if he did not pay yet. Tosfos and the Rosh hold that in every case the buyer must bring a proof, even if he did not pay yet. The problem was found here (in the buyer's Reshus), so we assume that it arose here. The Rambam agrees.

Beis Yosef (DH Kasav): R. Yerucham says that this refers to one who sold his own animal. If a wholesaler buys and sells, he swears that he did not know, and he is exempt. This is based on Bava Metzi'a 42b. This requires investigation whether we can distinguish between here and there.

Shulchan Aruch (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.

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.

Shulchan Aruch (2): If a needle was found in the Beis ha'Kosos and went all the way through, if there is no scab, it is a Safek. The buyer must bring a proof, for the Safek arose in his Reshus. If he does not bring a proof, he must pay the seller.

SMA (9): All agree to this law. According to the first opinion in Sa'if 1, it is because the hole was found here (in the buyer's Reshus), so we assume that the hole arose here. The latter opinion holds that since the Safek arose only after the buyer did Meshichah, he must bring a proof.

Taz (224:1,2): The Rif and Rambam explain that the 'Reshus' in which the Safek arose refers to location. The latter opinion in Sa'if 1 is Tosfos and the Rosh, who explain that it refers to ownership. If so, regarding a Safek when a calf was born, the one who bought the mother should need to bring a proof! Tosfos answered that it is different when the Safek favors the one in whose Reshus it arose. They argue about Meshichah to a Simta or joint Chatzer. According to the Rif, since the location does not belong to the buyer (the Safek did not arise in his Reshus), he is exempt. According to the Rosh, he is obligated. The Poskim did not mention this, but it is correct.