Gemara

(Mishnah): If Reuven's animal fell into Levi's garden and benefited, Reuven pays the benefit (not the full damage).

(Rav): The case is, it fell on fruits.

Rav teaches that Reuven pays the benefit not only when it ate, rather, even when it fell on fruits. We do not say that Reuven is exempt because the fruits were merely 'Mavri'ach Ari (chased away a lion).'

Question: Why don't we say this?

Answer #1: Mavri'ach Ari is only when Levi intended to prevent damage. Here, he did not.

Answer #2: Mavri'ach Ari is only when there is no loss. Here, there is a loss.

115b (Mishnah): If a flooding river overcame the donkeys of Reuven and Levi, and Levi neglected his own donkey (worth 100) to save Reuven's (worth 200), he is paid only like a worker (he is not compensated for the loss of his donkey).

If he said 'I will save yours and you will pay for mine', Reuven must pay him.

Nedarim 33a (Mishnah): (Even) if Reuven is Mudar Hana'ah from Shimon, Shimon may pay Reuven's debt.

Inference: This is considered Mavri'ach Ari.

Rishonim

The Rif (Bava Kama 24b, 41b,) brings the Gemaros in Bava Kama.

Rambam (Hilchos Nizkei Mamon 3:11): If Reuven's animal slipped on a rock or its urine and fell into Levi's garden and fell on produce, Reuven pays the benefit. I.e. it landed on a soft place, and its limbs were not crushed.

Hagahos Maimoniyos (2): This is not Mavri'ach Ari, because Levi did not intend to save it. Alternatively, it is because here there was a loss. Why is this different than paying another's loan, which is Mavri'ach Ari? Here, Levi lost and avoided a certain loss (Reuven's animal would have been hurt). Paying a loan did not avert a certain loss. Perhaps he would have persuaded the lender to pardon the debt. If one spilled his wine or abandoned his donkey to save another's honey or donkey, he is paid (only) for his toil. If one hired shepherds to fend off a lion, he is paid (back what he paid) up to the value of what he saved. This is because he prevented a sure loss. Why does one who hired shepherds receive more than the one who spilled his wine? This is an enactment to encourage people to save from the lion; we discuss when the owner was not there. The owner of the honey or other donkey was there; if not, his honey or donkey was Hefker, and the one who saved it could keep it. Therefore, Chachamim did not need to enact for him.

Rosh (Bava Kama 6:6): The two answers go together. Mavri'ach Ari is when Levi intended to prevent damage, therefore it is even if he suffered a loss. We find that one who paid a loan is called Mavri'ach Ari. Alternatively, Mavri'ach Ari is even without Levi's intent, rather, Reuven forced him to do so, but only if Levi did not suffer a loss. It is not Mavri'ach Ari only if it is without Levi's' intent and he loses, like our Mishnah. Reuven need not pay even for Levi's toil, i.e. when it is not clear that Reuven would have lost, e.g. the lion is far away, and Levi merely saves Reuven from worry. However, if it is almost certain that Reuven would lose, or he saves him from the lion itself, he gets his wages. If Levi spent money, Reuven pays everything. If one neglected his own donkey to save another's, he is paid only like a worker. This shows that he is paid for his toil; he is not a Mavri'ach Ari.

Tosfos (58a DH Iy): Likewise, one who returns an Aveidah is not Mavri'ach Ari.

Yam Shel Shlomo (6:13): Do not say that in every case letter of the law one is exempt, and every case in which one pays is an enactment so people will save others from loss (except for loans, in which there is no need to enact). This is wrong. If Levi intended to save Reuven's animal, he gets nothing.

Rosh (ibid.): The Mishnah (57b) discusses when falling on produce saved the animal from pain. Its value would not have decreased, but Reuven does not want his animal to be pained. Therefore, had Levi intended to save it, Reuven would have been exempt.

Yam Shel Shlomo (ibid.): It is astounding that one would pay to save his animal from pain without his knowledge. Earlier, the Rosh said that the produce saved the animal from harm! This is correct, like the Rambam says. Still, Tosfos properly says that the Mishnah does not discuss when it would have surely been harmed, for then surely Reuven pays exactly the damage saved. The Mishnah would be no Chidush! Rather, it is when it was a Safek whether it would be damaged. The Chidush is that we do not say that this is Mavri'ach Ari, even though it is very similar. Reuven pays what he would have given to save his animal from Safek damage and from pain. It was not enough for the Ri to say that the animal is saved from Safek damage, for the Mishnah says 'what it benefited', i.e. the animal's benefit is included. However, if we know that no damage would have resulted, obviously Reuven is exempt. When the damage was certain or almost certain, Reuven pays what he saved. He does not add for saving the animal from pain.

Rosh (ibid.): One who pays Reuven's loan is Mavri'ach Ari, for he does not avert a loss. Reuven was obligated to pay! He is saved only from pain (that he cannot pay). The Yerushalmi says that Reuven could have appeased the creditor.

Tosfos (ibid.): Alternatively, perhaps he would have found friends to pay the loan for him.

Poskim

Shulchan Aruch (CM 77:1): If two borrowed together and one of them paid the entire loan, he collects from the other what he paid for him.

Rebuttal (Shach 5): We hold that two who borrowed are merely Arevim (regular guarantors). They are not Kablanim (a lender can demand payment from a Kablan before demanding from the borrower). Therefore, paying the other's share is like a stranger who paid a loan. Since it was voluntary, this is Mavri'ach Ari, and the borrower need not repay him.

Defense (Gra): Here, it is as if each authorized the other to pay for him.

Shulchan Aruch (394:1): If Reuven's animal slipped on a rock or its urine and fell into Levi's garden and fell on produce, Reuven pays the benefit. I.e. it landed on a soft place, and its limbs were not crushed.

Shach (1): The Shulchan Aruch connotes that if there was no (concern for) crushing of limbs, only pain, Reuven is totally exempt. If there is a Safek, he must pay what he would have given to save his animal from Safek damage. Tosfos and the Rosh connote otherwise.