1) TWO OXEN WHICH CAUSED DAMAGE OF EQUAL VALUE TO EACH OTHER
QUESTION: The Mishnah states that if two oxen cause damage to each other and the damages are assessed to be of equal value, no payment is rendered. If the damaged caused by one is more than the damage caused by the other, its owner must pay the difference.
TOSFOS (DH Shnei) is bothered by a question. It seems obvious that if the damages are equal, the two owners should not have to pay each other equal amounts. Similarly, it seems obvious that if there is a difference between the values of the damages, only the difference has to be paid. Why is it necessary for the Mishnah to teach these obvious laws?
ANSWERS:
(a) The ROSH (3:13) answers that RASHI (DH Mu'ad) understands that even in a case of a Tam and a Mu'ad who gored each other, the calculation of the value of the damages is made based on the entire amount of the damage, even though the owner of the Tam normally pays for only half of the value of the damage. Without the Mishnah, one would have thought that the calculation is based on the amounts which each owner is liable to pay, and not on the actual value of the damage.
For example, if Reuven's Tam ox and Shimon's Mu'ad ox were fighting, and Reuven's ox caused forty Dinar of damage while Shimon's caused fifty Dinar of damage, normally Shimon would owe Reuven thirty Dinar -- fifty Dinar minus the twenty Dinar's worth of damage (Chatzi Nezek of forty Dinar) which Reuven should have to pay. However, the Mishnah teaches that Shimon owes Reuven only ten Dinar, the difference between the actual value of each ox's damage.
The GIDULEI SHMUEL points out that if one of the animals is owned by someone who has no liability at all, such as a minor, this would not be the case. Alternatively, if one of the oxen was being watched by a caretaker who accepted responsibility for any damage the ox causes, but not for damages that it suffers, this law would not apply. The caretaker would have to pay for the damages the ox causes, but the payment from the owner of the other ox would go directly to the actual owner of the ox in the care of the caretaker. Therefore, there would be no cancellation of payments of damages.
(b) TOSFOS clearly disagrees with the Rosh. Tosfos says that the net payments cancel each other out, and not the amount of actual damage. The Rosh quotes Tosfos as saying that the Mishnah indeed expresses no novel teaching.
(c) According to our text of Tosfos, Tosfos explains that the Mishnah indeed teaches a novel point according to both Rebbi Akiva and Rebbi Yishmael (end of 33a). According to Rebbi Akiva, who rules that an ox that damages may even be declared Hekdesh by the victim (since one collects "mi'Gufo"), one would think that the full value of the damage done by the Tam is automatically on lien to the victim. For example, in a case where two oxen attack each other and the Tam does fifty Dinar worth of damage, one might think that the victim may declare as Hekdesh twenty-five Dinar worth of the ox. The Mishnah therefore teaches that if the other ox did twenty-five Dinar worth of damage to the Tam, the damages are canceled out and the victim has no ability to be Makdish the Tam.
Tosfos adds that the Mishnah may be teaching an important point according to Rebbi Yishmael as well. Since a Tam pays "mi'Gufo" -- "from its body," one might have thought that in a case where two oxen damage each other and one of the oxen runs away, the owner of the runaway ox has an interesting claim: he may claim that he should still receive the damages done to his ox, while he does not have to pay for any damages that his ox did because his ox ran away and its value cannot be collected. The Mishnah therefore teaches that the damages are canceled out, meaning that the owner of the other ox automatically deducts the damage done to his ox even though the other ox is not present to collect the damages "mi'Gufo." (Mordechai Zvi Dicker)

33b----------------------------------------33b

2) SELLING A "TAM" THAT GORED
OPINIONS: The Gemara quotes Rav Nachman who says that a Tam that gored may not be sold, even according to Rebbi Yishmael who rules that its body is not actually owned by the victim. The Gemara asks that a Beraisa seems to contradict Rav Nachman's statement. The Beraisa explicitly says that if one sells the ox, the sale is valid.
The Gemara answers that the Beraisa means that if the owner sold his ox for plowing, the sale is valid even though the sale is not permanent.
What does the Gemara's answer mean?
(a) The RASHBA explains that the owner is allowed to sell the animal for plowing, and it is not considered stealing from the victim. The animal reverts to the possession of the victim only when Beis Din decides that this is the appropriate verdict.
(b) RASHI (DH l'Ridya) explains that if the buyer used the ox for plowing, he does not have to pay the victim for this usage. He merely must give the ox back to the victim.
The Rashba has difficulty with Rashi's explanation. Rashi implies that according to Rebbi Akiva, who rules that the animal is already considered to belong to the victim, the buyer would have to pay the victim for the value of the plowing. However, this clearly is not the law. The Gemara later (96b) states that if one steals an animal and plows with it, he does not have to pay the owner for this usage. He is required merely to return the stolen object. A thief is never obligated to pay for the extra benefit he received from the object he stole. Even when the buyer does not know that the ox he purchased is a stolen one, he still has the status of a thief who must return merely the actual object and not the value of the benefit received. Accordingly, there is no reason why one should think that he should have to pay according to Rebbi Akiva.
The Me'iri defends Rashi's explanation. In this case, the buyer of the ox is not considered a thief. The entire purchase is considered a "Mekach Ta'us" -- "a mistaken purchase," since the buyer would not have bought the ox had he known that it was stolen. Accordingly, the ox goes back to the owner and he must pay for the value of the usage of the ox (as indeed is the law in a case of Mekach Ta'us).
(It should be noted that the Gemara seems to understand that this sale for plowing is valid only according to Rebbi Yishmael. According to Rebbi Akiva, the victim already owns enough of the ox to make it Hekdesh, and thus the sale made by the original owner is invalid. This indeed is the opinion of TOSFOS (33a, DH Hikdish), and it also seems to be the simple explanation of the Gemara, as the Gemara asks its question only according to Rebbi Yishmael.
However, the RAMBAM (Nizkei Mamon 8:6) understands that the sale is valid for plowing even according to Rebbi Akiva. Many commentators discuss why the Rambam understands that the sale is valid even according to Rebbi Akiva. The LECHEM MISHNEH answers that although the Gemara's question is according to Rebbi Yishmael, since only Rebbi Yishmael would maintain that the sale is totally valid (while according to Akiva the victim already owns half of the animal), nevertheless even Rebbi Akiva agrees that the sale is valid for plowing (based on the explanation above). See also CHAZON ISH to Bava Kama 3:17.) (Mordechai Zvi Dicker)