Gemara

(Beraisa): If Reuven claimed 'you killed my ox' (or) 'you cut my young trees', and Shimon answered 'you told me to do so', he is exempt.

Objection (Rav): Surely, a damager cannot exempt himself by saying this!

Answer (Rav): Rather, the ox or tree needed to be killed or cut. Reuven wanted to do the Mitzvah himself.

(Beraisa): "He will spill (the blood of a wild animal or bird) and cover" - the one who slaughters covers;

R. Gamliel once obligated a man to pay 10 gold pieces for covering the blood of what another man had slaughtered.

Rishonim

Rif and Rosh (8:15): R. Gamliel made a man pay 10 gold pieces. This is letter of the law, and we learn from it to other cases.

Question: What was Rav's question? Perhaps one is believed to say that he damaged with permission when there are no witnesses, Migo (since) he could say that he did not damage!

Answers #1,2 (Nimukei Yosef): Surely, the Beraisa teaches more than just Migo. The Ro'oh says that we do not believe a Migo against a Chazakah, i.e. normally people do not permit others to damage their property.

Answer #3 (Mordechai 96): We can testify that he was not authorized to damage. However, if witnesses did not know that a loan was not paid, he is believed through the Migo to say that he was told to exempt the borrower. We cannot testify that this is not true (for if he was paid, it is reasonable).

Rambam (Hilchos Chovel u'Mazik 7:13): If the ox or tree needed to be killed or cut, because it was damaging people, and Shimon slaughtered or cut it without Reuven's consent, he must pay Reuven like the judges deem proper, for he prevented Reuven from doing a Mitzvah.

Magid Mishneh: The Rashba says that we do not collect this in Bavel, for judges in Chutz la'Aretz judge only cases with monetary loss. However, if he seized, we do not force him to return it.

Rambam (14): Similarly, if Reuven slaughtered a wild animal or bird, and Shimon covered the blood without Reuven's consent, he pays like the judges deem proper. Some say that there is a fixed fine of 10 gold pieces whenever Ploni prevents David from doing a Mitzvas Aseh that it was proper for David to do.

Rosh (8:15): A damager cannot exempt himself by saying 'you told me to do so', for surely one does not tell another 'damage my property.' Rather, the ox was a gorer that one may not keep alive, and the tree needed to be cut, e.g. it leaned over Reshus ha'Rabim or was an Asheirah (planted for idolatry).

Poskim

Shulchan Aruch (CM 382:1): If Reuven's ox or tree needed to be killed or cut because it was damaging people, and Shimon slaughtered or cut it without Reuven's consent, he must pay Reuven like the judges deem proper, for he prevented Reuven from doing a Mitzvah.

Beis Yosef (DH u'Mah): We discuss an ox that killed, and an Asheirah or a tree leaning over Reshus ha'Rabim that is prone to fall and kill someone.

Gra (1): We do not discuss an ox sentenced to be stoned, for it must be killed in Beis Din. Rather, even nowadays we kill animals established to gore.

Shulchan Aruch (ibid.): If Shimon says that Reuven told him to kill it or cut it, since it needed to be killed or cut, Shimon is exempt.

Beis Yosef (ibid.): Shimon is believed to say 'you told me to do so.' Presumably, it is true. Since it needed to be killed or cut, Reuven was not so particular, and forgot (that he authorized Shimon).

Rema: Other damagers are not believed to say 'you authorized me.' This is even if one has a Migo to say 'l did not 'damage', i.e. there are no witnesses, for it is a Migo against a Chazakah, which is invalid.

Gra (6): The Gemara (Bava Basra 6a) did not settle whether a Migo is believed against a Chazakah. It is not believed against a strong Chazakah.

Rema: He is believed to say 'I did not damage', Migo he could say 'I damaged my own property.'

SMA (4): If he says that he did not damage, and there are no witnesses, he is exempt without a Migo! Rather, it is clear from the Darchei Moshe (1) that the Rema teaches like the Mordechai (above). One is believed to say that he was authorized to damage only if he could claim that he owned what he damaged.

Shach (2): The Rema means that he is believed to say 'my action did not harm you.' E.g. he pardoned a debt, and says 'the borrower would not have paid, anyway.'

Shulchan Aruch (ibid.): Similarly, if Reuven slaughtered a wild animal or bird, and Shimon covered the blood without Reuven's consent, he pays like the judges deem proper. Some say that there is a fixed fine of 10 gold pieces for preventing David from doing a Mitzvas Aseh that it was proper for David to do.

SMA (5): One pays only for Mitzvos like covering the blood, which is an absolute obligation. If Ploni was given an animal to slaughter and Almoni slaughtered it, he is exempt, for one must slaughter only if he wants to eat.

Tur (YD Sof Siman 28): If Shimon took Reuven's Mitzvah, it is not enough for Shimon to give to Reuven a chicken to slaughter. The first Mitzvah is gone; "one cannot straighten the crooked." However, if Ploni was called to read the Torah, and Almoni came first and read, he is exempt, for Kri'as ha'Torah is obligatory for everyone. Even if a Yisrael read in place of a Kohen, he is exempt. A mere Asmachta teaches that a Kohen reads first.

Beis Yosef (Sof Siman 28): This comment is not from the Tur.

Prishah (4): Perhaps the Chidush is that one pays even for Kisuy, even though it is not certain that Shimon deprived Reuven of a Mitzvah. Perhaps the wind would have covered it before Reuven! We can explain the Tur to mean it is not enough to give him a chicken 'to slaughter and cover its blood.'

Tur (ibid.): If Reuven slaughtered and did not cover the blood, and Shimon saw this, he must cover it. This is like others Mitzvos obligatory on all Yisrael, just the slaughterer has precedence. If (Reuven wanted to cover it and) Shimon covered, he must pay him 10 gold pieces, the price of the Berachah that he made Reuven lose. We do not collect this nowadays.

Taz (28:8): If only the slaughterer were commanded to cover, if someone else covered he would not bless, and would not pay 10 gold pieces. That fine is only when another benefited, i.e. blessed. Here, he just damaged, i.e. deprived Reuven of a Mitzvah and Berachah. Rather, everyone is commanded to cover, therefore Shimon blesses, and pays 10 gold pieces. Semak says that even though we do not collect the fine nowadays, Shimon acted improperly and he must appease Reuven.

Rema: If Reuven had a son to circumcise and Shimon circumcised him, he must pay 10 gold pieces. If Reuven gave his son to Ploni to circumcise, and Shimon circumcised him, he is exempt.

Shulchan Aruch (ibid.): Nowadays, we do not collect this. If he seized it, we do not force him to return it.