Gemara

(Mishnah): If Reuven had two slaves (or fields), one big and one small, Shimon says, 'I bought the big one'; and Reuven says 'I sold the small one', Reuven swears that he sold the small one.

Question: Why does Reuven swear? His admission is not part of what Shimon claimed, it is Heilach (he gives him the small slave (or land) he admits to at the time of the admission), and we do not swear about slaves (or land)!

Answer #1: They argue about the value of a big or small slave or field.

Answer #2 (Shmuel): They argue about (fabric to make) clothing of a big or small slave, or the piles of grain of a big or small field.

Bava Kama 35b (Mishnah): If two oxen of Reuven damaged two of Levi's oxen, and Levi says that the big ox (or the Mu'ad) damaged the big ox, and the small ox (or the Tam) damaged the small ox, and Reuven says oppositely, ha'Motzi mi'Chavero Alav ha'Re'ayah.

(Rabah bar Noson): If David claimed that Shimon owes him wheat, and Shimon admitted that he owes barley, Shimon is exempt.

Question (Mishnah): If two oxen of Reuven, one big and one small...

(Inference): If Levi cannot bring proof, he collects like Reuven said.

Question: This is like wheat and barley (and R. Noson totally exempts)!

Answer: Rather, if Levi cannot bring proof, he should collect like Reuven says, but does not collect at all.

Question (Beraisa): Levi collects for the small ox from the big ox, and for the big ox from the small ox.

Answer: That is when Levi seized the damager.

Rishonim

Rif (Bava Kama 17a): We answered that Levi collects for the small ox from the big ox and for the big ox from the small ox when he seized them.

Nimukei Yosef (DH Echad): If Levi says that the Mu'ad gored the big animal, and Reuven says oppositely, he pays what he admits. This is unlike wheat and barley. Levi claims damage for the big one from the Aliyah (i.e. not limited to the value of the damager) and Reuven admits to damage for the small one from the Aliyah. This is like a claim for clothing (Shach's text - the value of) a big slave, and he admits to that of a small one.

Nimukei Yosef (DH Mai): When both are Tam, since the admission is not considered mi'Min ha'Ta'anah (like the claim) to obligate an oath, it does not obligate paying. One might have thought that admission and denial of wheat and barley is like admission and denial of their value, and he must pay the value of the barley.

Rambam (Hilchos Nizkei Mamon 9:11): If two oxen of Reuven damaged two of Levi's oxen, and Levi says that the big ox (or the Mu'ad) damaged the big ox, and the small ox (or the Tam) damaged the small ox, and Reuven says oppositely, ha'Motzi mi'Chavero Alav ha'Re'ayah. If there was no clear proof, Reuven is exempt. This is like a claim of wheat and admission of barley. He swears Heses and is exempt even from paying the barley.

Rosh (Bava Kama 3:15): The Mishnah said ha'Motzi mi'Chavero Alav ha'Re'ayah, and not 'exempt', to teach that if he seized the animals, he receives like Reuven said.

Poskim

Shulchan Aruch (CM 400:4): If two oxen of Reuven damaged two of Levi's oxen, and Levi says that the big ox (or the Mu'ad) damaged the big ox, and the small ox (or the Tam) damaged the small ox, and Reuven says oppositely, ha'Motzi mi'Chavero Alav ha'Re'ayah. If there was no clear proof, Reuven is exempt. This is like a claim of wheat and admission of barley. He swears Heses and is exempt even from paying the barley.

Gra (14): The Rashba says that when one of the damagers was Mu'ad, Reuven pays because he cannot swear. The Nimukei Yosef says that even this case is like wheat and barley. The Rambam holds that it is wheat and barley only to exempt Reuven for paying from the small one, but he must swear. Tosfos and the Poskim hold that whenever he did not need to know, he is totally exempt.

Question (Shach 10): When there are no witnesses, the Rambam and Shulchan Aruch exempt even when one of them was Mu'ad, unlike the Nimukei Yosef. Why is this different than an admission to a garment (or value) of a small slave in response to a claim of the garment (or value) of a big slave? In Bava Metzia and CM (95:3), we obligate for this! It is difficult to say that there, he never retracted from his admission. If so, the Gemara should have asked that he may retract, like it asked regarding Reuven's admission about which ox gored which!

Answer (Shach): The Rambam and Shulchan Aruch agree that an admission to the value of a small slave obligates. He cannot say that he was joking. We asked from Rabah bar Noson because he holds that an admission Lo mi'Min ha'Ta'anah is a mere rebuff. The same applies here. Presumably, Reuven was not there, and should have said 'I do not know about any damage.' Therefore, he can say 'I was joking', just like Rabah bar Noson says about an unnecessary admission to barley.

Rema: This is like the opinion I brought in Sa'if 3. Some distinguish and say that if there were witnesses that one damaged but they do not know which, Reuven pays according to what he says. This is not like wheat and barley; rather, it is an admission, like the first opinion in the Mechaber.

Shach (12): The Rambam (in Sa'if 3) connotes that the victim is Vadai. He adds that when the victim says that the damager knows, the damager swears mid'Oraisa and pays like he admitted. There is no Shevu'ah mid'Oraisa when the claimant is unsure!