Gemara

(Beraisa): If a shepherd was grazing the flock, and a lion came and killed some, we estimate. If he could have saved them, he is liable.

(Rabah): If a Shomer Chinam could have gathered for free shepherds with sticks to fend it off, he is liable. A Shomer Sachar must hire people.

Question (Abaye): A Shomer Sachar is exempt for Ones. Why must he pay to avoid Ones?

Answer (Rabah): The owner reimburses him.

Bava Kama 57b (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.

Rishonim

The Rif (Bava Kama 24b, 41b, Bava Metzia 54a) brings the Gemaros in Bava Kama, and the Gemara in Bava Metzia (in the name of Rav Yehudah).

Rambam (Hilchos Sechirus 3:6): If a shepherd could have saved animals from being torn or captured through (calling to) other shepherds and sticks, and he did not, he is liable, whether he is a Shomer Sachar or Shomer Chinam. However, if a Shomer Chinam could not gather shepherds and sticks for free, he is exempt. A Shomer Sachar is obligated to hire shepherds and sticks to save the flock. The owner reimburses 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. 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 he would lose, or he saves him from the lion itself, he gets his wages. If Levi spent money, Reuven pays everything. This is clear from Bava Metzia 93b. If a shepherd did not gather shepherds to fight off a lion, he is liable. A Shomer Chinam must try to gather them for free, and a Shomer Sachar for pay. He gets back the money from his employer. This connotes that if a Shomer Chinam voluntarily gathered them for pay, this was praiseworthy, and he is paid back what he spent and for his toil. The same applies to other people. Since a Shomer Chinam need not hire people, he is like any person (who volunteered to fight off the lion). When Levi neglected his own donkey to save Reuven's, he is paid only like a worker. This connotes that he is paid for his toil. We do not say that he is a Mavri'ach Ari.

Hagahos Ashri: This is when Reuven was there and Levi did not stipulate with him. If Reuven was not there, he pays Levi what Levi lost. This is like one who returns an Aveidah; if he lost wages (while caring for the Aveidah), he receives what one would want to be paid to be idle, as opposed to his full wage for his (more wearisome) job.

Poskim

Shulchan Aruch (CM 128:1): If Moshe paid Levi's loan document without Levi's knowledge, even if there was a security and Moshe took it back, Levi need not pay him. He takes back the security for free. Moshe lost his money. Even if the creditor was pressing Levi to pay, (some texts - perhaps, or we say that) Levi would have appeased him, and he would have pardoned the debt.

Rema: This is primary, unlike those who obligate Levi to pay. Some say that this is only when the lender is a Yisrael, but if he is a Nochri, all obligate Levi to pay, and all the more so if Moshe retrieved Levi's security from the Nochri. It is proper to rule like this.

Rebuttal (Shach 6): The Darchei Moshe cites this in the name of the Maharil. The Maharil does not say this. Also, it is against the Poskim! The Mordechai obligates only when Levi did not really owe. This is even if the lender was a Nochri. A Nochri creditor is no worse than a pressing creditor. The Yerushalmi learns from Shekalim, which no one can pardon. It is Mavri'ach Ari because Levi can say that he would have found friends to pay for him (Tosfos). This applies equally when the creditor is a Nochri. If Levi did not really owe, then he must pay Moshe, for friends do not pay a thief.

Rema: Some say that this is when Moshe did not have anything of Levi, but if he had Levi's money and paid the debt with it, Levi has no claim against him.

Rebuttal (Maharam Lublin 22, cited in Shach 8). Levi is exempt because he could have appeased the creditor. This is even if Moshe had money of Levi!

Defense (Shach 8): Levi is exempt because Moshe was Mavri'ach Ari (Tosfos Bava Kama 58a DH Iy). We assume that he intended for a Mitzvah. If he had Levi's money, we do not say so. Actually, the Rema's law is obvious, for R. Noson's law applies mid'Oraisa even if Moshe has his own money.

Shulchan Aruch (CM 303:8): If a shepherd could have saved animals from being torn or captured through (calling to) other shepherds and sticks, and he did not, he is liable, whether he is a Shomer Sachar or Shomer Chinam. However, if a Shomer Chinam could not gather shepherds and sticks for free, he is exempt. A Shomer Sachar is obligated to hire shepherds and sticks to save the flock. The owner reimburses him.

Shach (8): Also a stranger who paid to save it is paid back.