Gemara

(Rav Hamnuna): If a Kohen took a Safek Bechor, we do not force him to return it, for ha'Motzi mi'Chavero Alav ha'Re'ayah (perhaps it is truly a Bechor).

Rejection (Rabah): Perhaps we force the Kohen to return it.

Support (R. Chananya - Beraisa): (A Seh used to redeem) a Safek (Peter Chamor, i.e. firstborn donkey) is tithed with other animals.

If a Kohen who took a Safek may keep it, how could a Yisrael make it Ma'aser? He exempts his obligation with the Kohen's property!

Rejection (Abaye): Perhaps if a Kohen took a Safek he must return it. The case is, there are only nine other animals. Even if the Safek really belongs to the Kohen, the Yisrael has no obligation to take Ma'aser!

Retraction (Abaye): This is wrong. A Safek is not tithed.

102b: A case occurred in which Levi rented a bathhouse to David for 12 Dinarim for the year, a Dinar per month, and it became a leap year. R. Shimon ben Gamliel and R. Yosi ruled that David pays half for the added month.

(Shmuel): This is when they come to Beis Din in the middle of the 13th month. If Levi said at the beginning of the month 'you must pay extra for this month', David must pay for the entire month. If Levi did not claim until the end of the month, David is exempt.

Shmuel is unsure whether the latter phrase is binding, so we follow Chazakah. Levi is Muchzak in his property. David cannot use it without proof. David is Muchzak in his money. He need not pay for what he used without proof.

(Rav Nachman): Levi is Muchzak in his land (even if Levi claims at the end of the month, David must pay).

Kesuvos 19b (Beraisa): If David and Levi signed a document and died, and two witnesses recognize the signatures but say that David and Levi were coerced, children or invalid witnesses at the time, they are believed.

Question: Why do we rely on David and Levi more than the latter witnesses?

Answer (Rav Nachman): The latter witnesses counter the former two, and we leave the money (or property) in its Chazakah with its current owner.

Rishonim

Rambam (Hilchos Bechoros 2:6): If a cow gave birth to what resembles a horse or camel, even if it has some signs of a cow, it is a Safek Bechor. Therefore, the owner eats it. If the Kohen seized it, we do not take it from him.

Kesef Mishneh: Rav Hamnuna said that if the Kohen seized it, we do not take it from him. If the Kohen did not seize it, one may not shear it or work with it. However, Rabah rejected this, and Rav Chananya supported him from a Beraisa! It seems that the Rambam explains that Rabah gave a mere Dichuy (he showed that one could explain differently; he does not necessarily disagree). The support is not so solid. Even though we do not take it from him, since it is in the Yisrael's Reshus and ha'Motzi mi'Chavero Alav ha'Re'ayah, it is considered the Yisrael's money, and he can exempt himself through it.

Ri Korkus (DH Od): Rabah gave a mere Dichuy. He was unsure, so we rely on Rav Hamnuna, who was sure. We do not rely on Rav Chananya's support, for it is not so solid.

Rashba (1:311): The Gemara connotes that the Halachah follows Rabah, but we must strive to defend the Rambam. The Gemara concludes that ha'Motzi mi'Chavero Alav ha'Re'ayah. This connotes that either side can be Muchzak, like it says in Kesuvos (76b). The Yisrael is Muchzak. Rabah gave a mere Dichuy; he did not say 'there, if the Kohen takes...', rather, 'really, I can say...' He shows that the Isurim of shearing and working with it do not depend on whether or not we take it from a Kohen who seized it. Rav Chananya surely holds that we take it from the Kohen, but the Halachah follows Rav Hamnuna against Rav Chananya. Firstly, the Mishnah connotes like him. Also, whenever Amora'im argue and the Gemara did not resolve it, we do not take from one who seized. Rav Hamnuna can reject the proof from the Beraisa, for an automatic Kedushah is different. Even though the Yisrael owned the mother, he is no more Muchzak in the child than the Kohen is. This is like one who traded a cow for a donkey (through Chalipin), and we do not know if the cow gave birth before or after the sale. However, there both have Drara d'Mamona. Here neither has Drara d'Mamona, therefore, Kol d'Alim Gevar (whoever overpowers will take it). However, regarding a Safek Peter Chamor (firstborn donkey), the Kohen has no rights in it itself. The Seh used to redeem it is Muchzak to the Yisrael, so if the Kohen seized it, we take it from him. Abaye retracted to say that there is no support for or question against Rav Hamnuna. Therefore, the Halachah follows Rav Hamnuna.

Lechem Mishneh: The Rambam learns from Kesuvos 20a. When two pairs of witnesses contradict each other, we leave the property in its Chazakah. Rashi says that we do not tear the document, for if one seized, he keeps it. Tosfos asked why this is different than a Safek Bechor; if the Kohen seized it, we take it from him. Tosfos gave a weak answer. Due to this question, the Rambam concluded that we do not take it from the Kohen.

Rambam (5:3): Any Safek Bechor should graze until it gets a Mum, and the owner eats it. If the Kohen seized it, we do not take it from him.

Rambam (Hilchos Bikurim 11:19): If there is a Safek whether or not a person must be redeemed, he is exempt, for ha'Motzi mi'Chavero Alav ha'Re'ayah.

Radvaz: Regarding Safek Bechor Behemah, the Rambam rules that if a Kohen seized it, we do not force him to return it. The conclusion in Bava Metzia connotes that we force him to return it, and Tosfos and the Rosh rule like this; elsewhere, I defended the Rambam. The Rambam did not say whether or not we force him to return Safek redemption of Bechoros of people and donkeys. It seems that he agrees regarding people, for the Kohen did not seize the matter about which there is a Safek.

Machaneh Efrayim (Zechiyah u'Matanah 8): The Rambam rules that if a Kohen seized a Safek Bechor Behemah, we do not force him to return it because he holds that Tovas Hana'ah (the right to choose to which Kohen he will give) is not Mamon (a monetary privilege of the giver). We do not force him to return it only regarding Terumah and Matanos Kehunah, for once they are separated they are property of Kohanim. The five Sela'im for Pidyon ha'Ben are the father's money until he gives them. The Kohen has no rights to them, just there is a lien on the father's property. If he seized them, it is theft. The father can say that he will give to another Kohen. If a Kohen seized Safek Matanos Kehunah, he can say that he seized money of Kohanim. If the Yisrael wants to take it back, he is ha'Motzi mi'Chavero Alav ha'Re'ayah (he must prove that they are not Matanos).

Rosh (Bava Metzia 1:13): Really, if a Kohen took a Safek he must return it. Seizure helps only for one who has a definite claim. Rav Nachman rules (when there is a Safek about the extra month) that seizure does not help because the one who seizes is unsure. Even the opposing opinion does not say that seizure helps. Rather, being quiet until the end of the month is admission that he rented it for the entire year.

Rosh (Bechoros 1:15): The Gemara did not resolve whether one may redeem a Peter Chamor with a baby that does not resemble its mother. If the (baby) donkey died before giving it to the Kohen, he is exempt. If he gave it to the Kohen and then the donkey died, the Kohen keeps it, for ha'Motzi mi'Chavero Alav ha'Re'ayah. We do not take from one who seized b'Reshus (with permission), but seizure (without Reshus) by force does not help.

Ramban (Hilchos Bechoros 3a): We concluded in Bava Metzia that if the Kohen seized a Safek Bechor, we take it from him.

Poskim

Shulchan Aruch (YD 315:1): One need not give a Safek Bechor to a Kohen. The owner keeps it until it gets a Mum, and eats it. One may not shear it or work with it. If the Kohen seized it, we do not take it from him.

Rema: Some say that we take it from him. This is the Halachah. Even if the Yisrael gave it mistakenly, e.g. he thought that he is obligated to give it, and later found out that he did not need to, the Kohen must return it.

Taz (1): The Kohen's seizure does not help, for he was not allowed to seize it due to the Safek.

Shach (2,3): The first opinion holds that after seizure, the Kohen is Muchzak and the Yisrael is ha'Motzi mi'Chavero. The latter opinion holds that the Yisrael was Muchzak in Tovas Hana'ah, i.e. to give it to any Kohen he wanted, therefore, he is Muchzak in the animal, wherever it is, and the Kohen is the Motzi.

Gra (2): Most Poskim hold that we take it from the Kohen. Some say that the Hava Amina in Bava Metzia was that we discuss a Safek Bechor, but the conclusion is that we discuss a Safek Peter Chamor. The Kohen has no rights in the donkey, and the Seh used to redeem it is Muchzak to the Yisrael, so if the Kohen seized it, we take it from him him. The Rambam holds that ha'Motzi mi'Chavero Alav ha'Re'ayah applies to all Sefekos. After one seized, the other is ha'Motzi. Bava Metzia 105b proves this. Rav Nachman disagrees only due to the Chazakah of land. The Rambam's opinion is primary.