Gemara

(Mishnah): If one of Reuven's oxen damaged Levi's ox, they are liable. If Levi says 'the big (or Mu'ad) ox damaged', and Reuven says 'no, the small (Tam) one damaged', 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: In the Mishnah, if Levi cannot bring proof, he collects like Reuven said. 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

The Rif brings our Mishnah.

Rambam (Hilchos Nizkei Mamon 9:9): If Reuven's two oxen chased Levi's ox and we do not know which of them damaged it, and Levi says 'the big one (or Mu'ad) damaged', and Reuven says 'the small (Tam) one damaged', ha'Motzi mi'Chavero Alav ha'Re'ayah.

Magid Mishneh: If he cannot bring a proof, Reuven is totally exempt. The Rambam says so in Halachah 11, when each damaged one animal. It is like an admission of barley in response to a claim of wheat. Further, since Reuven admits about a Tam, it there are no witnesses he is exempt like one who admits to a fine. Our Sugya is like the opinion that half-damage is Mamon. The Halachah does not follow this.

Rambam (10): If there was no clear proof, but witnesses say that one of them damaged, Reuven pays according to what he said. If Levi says 'you know that this one damaged in front of you', Reuven swears mid'Oraisa and pays like he admitted, for he partially admitted.

Ra'avad: This refers to the case of a Tam and a Mu'ad. The Mishnah says ha'Motzi mi'Chavero Alav ha'Re'ayah in any case when the damager says perhaps the Tam damaged; then, he need not swear. We do not say that he is obligated to swear and cannot, and therefore he must pay. We say so only when the other side has a Vadai claim and says 'you know.'

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.

Question (Rosh 3:15): Rashi and Tosfos say that since he claimed only wheat, he pardoned the barley. This is difficult. Often, a person has several claims against someone, and does not claim all at once! Also, Rabah bar Noson's law is unlike our Mishnah. Even if you will say that if David claimed that Shimon owes him wheat, and Shimon admitted that he owes barley, Shimon must pay barley because David's silence about it is not pardon, our Mishnah is not due to pardon. Rather, Levi agrees that only one ox gored his animal. Since he says that it was the big one, he admits that the small one did not gore. And even if our Mishnah obligated Reuven, for Levi admitted about the small one only in order to collect from the big one, we could say that in Rabah's case he is liable!

Answer (Rosh): We must say that Rabah discusses a case like our Mishnah. I.e. David claims 'on day Ploni at time Ploni I lent you a Kor of wheat, and Shimon says that it was barley. Had he also lent barley at the same time, surely he would have mentioned it. We must say that he pardoned the barley. We said that the Mishnah supports him, for it must discuss this case. If not, Shimon would be liable. One who makes a claim does not pardon all other claims, unless it was for the same time! The Gemara rejects the proof. Perhaps he owes barley, even if both claims were at the same time.

Rosh (citing the Ramah): Shimon's exemption is not due to David's pardon or admission. Rather, Shimon's admission does not obligate him. An admission obligates only if he said 'you are witnesses against me', or it was in front of Beis Din, after the other claimed from him. A Tosefta allows retracting from an admission that was not in response to a claim. Therefore, if David replied 'at least pay me the barley you admit to', Shimon can say 'I was jesting.' If he does not say this, he is liable.

Rebuttal (Rosh): If so, what was the question from our Mishnah? He cannot say that he was joking! Surely witnesses saw the goring, just they are unsure which ox gored. If not, he admits to a fine, and is exempt! Since Reuven claimed a large damage, Shimon was forced to say that he owes only a small damage. He did not jest! Rather, we must say that he is exempt due to Reuven's admission.

Poskim

Shulchan Aruch (CM 400:3): If Levi says 'your big (or Mu'ad) ox damaged', and Reuven says 'no, the small (Tam) one damaged', ha'Motzi mi'Chavero Alav ha'Re'ayah.

Shach (4): We do not say that he is obligated to swear and cannot, and therefore he must pay, because Levi does not say 'you know.' Here, Reuven was not present to see what happened!

Shulchan Aruch (ibid.): If Levi says 'you know that this one damaged in front of you', Reuven pays like he admitted and swears mid'Oraisa about the rest.

SMA (4): This refers to the case of a Tam and a Mu'ad. He is Modeh b'Miktzas because in any case Levi has a claim to collect from the Tam. The Rema rules like the Rosh, who exempts even in this case. When both are Tamim, all agree that he is totally exempt.

Shach (5): Why is there a Shevu'ah mid'Oraisa? Levi claimed a Mu'ad, and Reuven must pay a Tam, which is not mi'Min ha'Ta'anah (the same species claimed)! The Magid Mishneh says that a claim against a Mu'ad applies to all Reuven's property. I disagree. If Yakov claims that he lent 100 to Moshe, and Moshe admits to a Kli worth 50, is this Modeh b'Miktzas?! It is not, for it is not Mi'Min ha'Ta'anah (CM 88:13). Rather, when one claims money, anything is considered mi'Min ha'Ta'anah. If he claimed a gold Dinar, we say that he meant the value of a gold Dinar (88:9). However, a Tam pays only mi'Gufo, and if it is lost there is no compensation, so it is not considered mi'Min ha'Ta'anah. We ask against Rabah bar Noson, who holds that whenever the admission is not mi'Min ha'Ta'anah, he can say that he was joking.

Gra (8): Since he claims the money for the damage, this is mi'Min ha'Ta'anah. We compare it to Rabah bar Noson only regarding whether he pardoned or saying that he was joking. There is an oath only when Levi says that Reuven knows and Reuven claims Vadai. The Rambam learned this from the Gemara, which said ha'Motzi mi'Chavero Alav ha'Re'ayah when Reuven was unsure. We do not say that he pays because he cannot swear.

Rema: Some say that there is no oath here at all. This is like a claim of wheat and admission of barley. He is exempt even from what he admitted to. Even if he seized, we make him return it. This is only when Levi makes a Vadai claim. If he is unsure, Reuven pays according to his admission.

Shach (8): I proved (88:16,22) that the Halachah does not follow this opinion, Whenever there are witnesses, he pays the smaller, and Tefisah helps.

SMA (3): The Mechaber rules like the Rambam. The Rema brings the opinion of the Rosh and Tur. However, he should have said 'he does not pay at all', like the Tur. Also, he does swear Heses! We must say that the Rema means that he is exempt from the payment and Shevu'ah mid'Oraisa that the Rambam obligates. He does not discuss Shevu'as Heses.

Beis Yosef (DH u'Mah she'Chosav Rabeinu): The Rambam obligates Reuven to pay according to what he said, even when Levi claims Vadai. The Tur questions this, for this is like wheat and barley. The Magid Mishneh explained that here is different, for there are witnesses that one of them damaged.