Gemara

One is not liable (to pay) for hitting an animal unless he made a wound, for it says 'Nefesh' (which is in the blood).

Rejection (R. Yirmeyah): If so, if one weakens an animal (by overloading it) with rocks, he should be exempt!

Bava Kama 85b (Rabah) Question: If Reuven damaged Shimon, and Shimon will later return to full health, but because he cannot work now, he is worth less, what is the law? (E.g. his hand was stricken. It shriveled, but later it will heal fully.)

Since it will later return to normal, Reuven is exempt (from paying for the hand). Or, since Shimon is worth less now, perhaps Reuven is liable!

Answer #1 (Abaye): He pays both Nezek (the decrease in the slave-market value of the victim) and Sheves (compensation for inability to work until he recovers).

Answer #2 (Rava): He pays only Sheves.

Bava Metzia 96b - Question: If an animal became weaker due to working, what is the law?

Question (Rav Chilkiya brei d'Rav Avya): This implies that if it died due to working he is liable. This cannot be. He borrowed it to work with it!

Answer (Rava): Even if it died due to working, he is exempt. He borrowed it to work with it!

Rishonim

Rosh (Bava Metzia 8:4): If one rented an animal and through his negligence its foot was bruised and will later recover, Tosfos exempts. The Halachah follows Rava that one pays only Sheves for temporary damage, and Sheves is not paid for animals. If so, the question about an animal that weakened due to working must be when it will never recover. Others say that regarding people, temporary damage is considered Sheves, for people are not normally sold, but animals are normally sold, so it is Nezek. Also, people are sold for much money, and (proportionally) one would not deduct so much for such injuries. One would deduct much from the sale of an animal.

Tosfos (Gitin 42b DH v'Shor): Some exempt a renter or borrower who damaged an animal, even through negligence, if it will recover. "A man (who damages) his fellow man" pays five payments, but not an ox that damages an ox (Bava Kama 26a). It pays only Nezek. Likewise, a man who damages an ox pays only Nezek. We do not distinguish one who damaged from a Shomer, for the Gemara (Bava Kama 4b) calls Shomrim cases of man damaging animals! In Sanhedrin, we say that one who weakens an animal is liable. This refers to damage that will not recover.

Tosfos R. Peretz (Bava Kama 86a). R. Chaim Kohen says that the opinion that obligates for Garmi (actions prone to cause damage in a semi-direct way) obligates for temporary damage to animals, lest one who was angered do such damage to the one he resents.

Hagahos Ashri (Bava Kama 8:6): If one rented an animal and it was damaged, and does not recover for a month or two after returning it, even if it was through negligence he is exempt. Since it will recover, this is Sheves, and one who damages an animal pays only Nezek.

Poskim

Shulchan Aruch (CM 307:6): If one rented an animal and through his negligence it was bruised and will later recover, some exempt because this is Sheves, and Sheves is not paid for animals. Some obligate paying.

Beis Yosef (DH Kosav Adoni): Here the Tur did not favor either opinion. In Siman 340, he rules like the first opinion.

Bach (DH Kosav): This law applies also to any Shomer who was negligent. Why do they discuss a renter? Perhaps a case occurred with a renter and Rabanan argued about it, but indeed, they argue about all Shomrim.

Shevus Yakov (3:178): The renter must pay to heal the animal. If he did not heal it, he would need to pay for Nezek! What an animal eats while sick is Nezek, for beforehand it ate and worked, and now it eats without working. Sheves is the excess it used to work above the cost of its food.

Rema: The first opinion is primary.

Darchei Moshe (2): In Siman 340, the Tur brings only the first opinion. Also the Mordechai (Bava Kama 85,86) and Moharam rule like it.

Shulchan Aruch (340:2): If a borrowed animal became weaker by itself, and it will recover, the borrower is exempt.

Drishah (3 and SMA 307:10): The Rema favored this opinion because the Tur brings only this opinion here. This is unreasonable. Above, the Tur closed with the opinion that obligates (which shows that this is primary)! Also, here the Tur exempts only if it weakened by itself! Also, the Rosh closed with the opinion that obligates. Surely, the Tur agrees! Rather, the Tur exempts because it was Ones and it will recover. This is unlike "Nishbar Oh Mes" for which the Torah obligates. The Rosh and Tur say that some argue with Tosfos, i.e. only when it weakened through negligence. This is the Halachah.

Rebuttal #1 (Shach 5): In any case, this is Sheves! Rather, the Chiyuv to pay for weakening an animal is only when it will never recover. The Rosh said that the opinion that exempts temporary damage must say that the question about an animal that weakened due to working must be when it will never recover. According to the SMA, all must say so!

Defense (Ketzos ha'Choshen 340:2 DH u'Mah): The Hava Amina of the Gemara was that weakening due to working is like overt damage. Therefore, the stringent opinion would obligate even if it would recover.

Shach (5): R. Yerucham says that the opinions argue about weakening, just like about a wound. The Tur's Piskei ha'Rosh prove that he rules like the first opinion. The Darchei Moshe said that also Moharam exempts. This is wrong; he obligates. It is difficult to rule against Moharam. The Maharshal and Piskei ha'Tosfos said that one is liable. It seems that it is a Safek, so ha'Motzi mi'Chavero Alav ha'Re'ayah, especially since Tosfos, the Rosh and R. Yerucham did not decide. This is unlike the Rema and Ir Shushan, who rule like the first opinion. The Maharshal exempts for Sheves without Nezek, e.g. one locked an animal in a room so it could not work.

Rebuttal #2 (Taz): We do not distinguish a borrower's obligation to pay for Ones from a renter's obligation to pay for negligence. The Tur discussed when it became weaker by itself to teach the opposite, that if it weakened due to work he is exempt, but if it was not due to work, i.e. even through negligence, he is exempt, like Tosfos.

Note: I suspect that the Taz means as follows. The Tur exempts for temporary weakening also through work in every case, i.e. including negligence. Rather, the distinction is that if it weakened due to work (without negligence) and died, he is exempt, but not if it weakened by itself and died.

Ketzos ha'Choshen (307:3 and ibid.): Tosfos (Gitin 53b DH Gazlan) says that one who overtly weakens an animal is liable, but if it happened by itself a thief is exempt. This must be when it will not recover. If not, a thief would acquire it and need to pay! Also Tosfos (Bava Kama 56b DH Pshita) says that a Shomer can be liable when a thief is exempt, e.g. it weakened, but will recover. This shows that Tosfos calls such damage to an animal 'Nezek'.

Ketzos ha'Choshen (340:2): The SMA exempts temporary damage that happened by itself because it is Sheves. This is wrong. If so, even overt damage would be exempt! Rather, it is like the Shitah (Bava Metzia 96b DH Kichesh). The Shitah asked that a Shomer should not be more liable than a thief, who is exempt if it weakened. He answers that he overtly weakened it. He must discuss when it will recover, for otherwise a thief is liable, and he distinguishes between overt damage and what happened by itself.