Gemara

91b (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.

(Rav): A damager cannot exempt himself by saying this! 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.

Chulin 87a - Question: Is this for 'stealing' the Mitzvah, or for depriving him of the Berachah? This determines the fine for stealing Birkas ha'Mazon (when someone else should have blessed.) It is only one Mitzvah, but it is four Berachos.

Answer: Once, Rebbi invited a Tzeduki to eat, and offered him 40 gold pieces if he will decline to drink the Kos Shel Berachah (the wine over which they will say Birkas ha'Mazon). The man declined the money. A Bas Kol announced that 40 gold pieces is the value of Birkas ha'Mazon.

Bava Kama 116b: Once, Rav Nachman obligated a Moser to pay. Rav Huna bar Chiya asked if this was mi'Dina (letter of the law), or a fine.

Question (Rav Yosef): What difference does it make if it was mi'Dina or a fine?

Answer (Rav Huna): If it was mi'Dina, one may learn to other cases. If it was a fine, one may not.

(Beraisa): At first they used to obligate for being Metamei another's Peros, or Menasech (pouring wine to idolatry). Later, they included Medame'a (mixing Chulin with Terumah, forbidding it to a non-Kohen).

Suggestion: Initially, we could not learn Medame'a (or other cases of unnoticeable damage) from Metamei and Menasech, for they are fines,

Rejection: No. At first they fined only for a major loss (Tamei Terumah and Yayin Nesech are forbidden to all). Later, they fined even for a small loss.

Gitin 52b (Rav): Menasech is literal; he poured wine for a libation to idolatry.

(Shmuel): No, he mixed Shimon's wine with wine that was offered to idolatry.

Rav didn't explain like Shmuel, for mixing wine is just like mixing Peros with Terumah. The Tana would not teach this case twice.

Shmuel holds that we can not derive one fine from another. Both must be taught.

Rishonim

Rif and Rosh (Bava Kama 32b and 8:15): Some say that R. Gamliel levied a fine of 10 gold pieces, and we do not learn from it to other cases. The Gemara is unlike this. Rav inferred that if one was not told to cut the tree, he is liable. Rather, it is mi'Dina; we learn from it.

Shach (CM 382:1): Rav (Gitin 53a) holds that we learn from one fine to another! This is the conclusion, even though Shmuel disagrees.

Rosh: Also, if it were not a fine, why should we fine one who intended to do a Mitzvah?! Also, in Chulin we ask whether 10 gold pieces is for the Mitzvah or the Berachah, and this determines what one pays for Birkas ha'Mazon.

Ran (Chulin 29a): We do not collect this nowadays, for there is no monetary loss. The Rif wrote it because if he seized, we do not force him to return it, like all fines.

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.

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.

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.

Drishah (2 and SMA 1): In these cases the Tur obligates like the judges deem proper, and for covering the blood and other Mitzvos he obligates 10 gold pieces. It seems that in cases when it was a big damage and there was no time to ask Reuven, Shimon is exempt. However, the Rambam and Shulchan Aruch bring two opinions, whether the judges assess or if the fine is fixed. Neither opinion distinguishes. It seems that if Reuven is meticulous about Mitzvos, they levy a big fine. If not, they fine less. Also, if it is a big exertion, sometimes a person leaves it for others to do. The Tur merely wrote the text of the Gemara. The Tur says 'the same applies to any Mitzvah', i.e. even if there is no Berachah.

Question (Shach 1): Why does the Tur say that one pays like the judges deem proper? The Rosh holds that mi'Dina, he (always) pays 10 gold pieces!

Shiltei ha'Giborim (32b 3): The Rambam concludes that we learn from Kisuy ha'Dam to other Mitzvos. This shows that it is not a fine.

Rebuttal (Shach 1): The Rambam and Shulchan Aruch hold that it is a fine. If it were mi'Dina, obviously one must pay 10 gold pieces, like R. Gamliel obligated for Kisuy ha'Dam! Rather, it is a fine according to what the judge deems proper. The latter opinion holds that the fine is fixed. R. Yerucham (15:4) says that some say that 10 gold pieces is only for a Mitzvah with a Berachah. Others say that it is even for a Mitzvah without a Berachah, like we find regarding killing an ox or cutting a tree. The Rashba and Ran conclude that one pays 10 gold pieces for each Berachah, and also for a Mitzvah without a Berachah. We do not distinguish Mitzvos from each other.

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.

Gra (382:2): The Rif and Rosh rejected the first opinion, for if it is a fine, we cannot learn to other fines, but in Bava Kama (91b) and Chulin (87a), we learn to other cases. On 117a we rejected the proof that one may not learn from one fine to another. It is a mere Dichuy; really, we may not learn from one to another, like Shmuel. There is no question from Bava Kama, for there Rav learned to another case. We can say that also Chulin 87a is like Rav. The Rambam rules like Rav because these two Sugyos are like him.

Note: Perhaps the Rif and Rosh hold that it is better to explain the Sugyos even like Shmuel, for we rule like Shmuel against Rav in monetary laws.

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

Gra (8): This is like the latter opinion. (According to the first opinion, he could not keep what he seized, for Beis Din never decided how much to fine.)