Gemara

Question: Tana'im argue about a Shomer (one who guards an) Aveidah who sold the Aveidah and used the money. If he did not use it, all agree that if it was lost, he is exempt. This refutes Rav Yosef!

(Rabah): A Shomer Aveidah is considered a Shomer Chinam (for he gets no benefit. Therefore, he is exempt if it stolen or if he loses it);

(Rav Yosef): He is like a Shomer Sachar (he is liable for loss or theft).

Bava Kama 56b - Version #1: Rav Yosef holds that he is like a Shomer Sachar because the Mitzvah to guard an Aveidah exempts him from giving bread to an Oni (who requests).

Version #2: He is like a Shomer Sachar because the Torah obligated him to guard the Aveidah.

Answer (Rav Yosef): All agree that if the money was stolen or lost, he is liable. They argue about Ones. R. Tarfon permits him to use the money, so he liable like a borrower. R. Akiva forbids him to use the money, so he is exempt.

81b (Mishnah): If Reuven lent to Shimon (relying) on a security, Reuven is a Shomer Sachar on the security.

Our Mishnah is unlike R. Eliezer:

(Beraisa - R. Eliezer): If Reuven lent to Shimon (relying) on a security and lost the security, he swears (that he lost it) and collects the debt.

R. Akiva says, he lent only due to the security. If he lost it, he lost the loan.

82a - Suggestion: They argue about Shmuel's law, when the security is worth less than the loan.

(Shmuel): If Reuven lent 1000 Zuz and took an axe handle for a security, if he loses the handle, he loses the whole loan. (R. Akiva holds like Shmuel. R. Eliezer argues, for the security was only a remainder of the loan.)

Rejection: All disagree with Shmuel. In such a case, all agree that he collects. The Tana'im discuss a security taken at the time of the loan. They argue about whether a Shomer Aveidah is a Shomer Chinam or a Shomer Sachar.

Suggestion: Rav Yosef's law is like R. Akiva, and R. Eliezer disagrees.

Rejection: No, all can agree with Rav Yosef. They argue about when the lender needs the security. R. Akiva says that we still consider the loan to be a Mitzvah, therefore he is a Shomer Sachar (for Rav Yosef's reason).R. Eliezer does not consider it to be a Mitzvah, so he is a Shomer Chinam.

Rishonim

Rif (Bava Kama 24a): The Halachah follows Rav Yosef, for the Gemara in Bava Metzia (82a) and Shevuos (44b) say that all hold like him.

Rif and Rosh (Bava Metzia 51a and 6:16): We hold like our Stam Mishnah, that one who lends on a security is a Shomer Sachar, because he did a Mitzvah. He is exempt from giving bread to an Oni, for Osek b'Mitzvah Patur Min ha'Mitzvah. This is why Rav Yosef says that a Shomer Aveidah is like a Shomer Sachar. The Halachah follows him.

Rambam (Hilchos Gezeilah 13:10): As long as the Aveidah is with the finder, if it was stolen or lost he is liable. If Ones occurred, he is exempt. A Shomer Aveidah is like a Shomer Sachar. Since he is engaged in a Mitzvah, he is exempt from several Mitzvos Aseh as long as he is engaged in guarding the Aveidah.

Rosh (Shevuos 6:29): The Halachah follows Rabah. He holds (Bava Metzia 29a) that if the finder sold the Aveidah, he is a Shomer Sachar on the money because he may use it. Rav Yosef holds that he is a borrower. The Halachah follows Rav Nachman in Mamon, and he says (Bava Metzia 43a) that a moneychanger is a Shomer Sachar on money deposited with him, because he may use it!

Rebuttal: Rabah holds that one who guards the Aveidah itself is a Shomer Sachar. If he sold it, the rights to use the money raise his obligation to be a Shomer Sachar. Similarly, Rav Yosef holds that one who guards an Aveidah is a Shomer Sachar, so the rights to use the money make him a borrower!

Rejection of Rebuttal (Rosh): Rav Yosef says that a Shomer Aveidah is a Shomer Sachar due to the exemption from Tzedakah. The same applies if he receives a second benefit (ability to use the money). The Chiyuv of a Shomer Sachar does not depend on how much he is paid!

Rosh: The Gemara suggested that Rav Yosef's law is like R. Akiva, and R. Eliezer disagrees. It did not mention Rabah. Some (Rashi Bava Metzia 82a DH Leima) say that this is because Rabah surely holds like R. Eliezer; R. Akiva cannot hold like Rabah. The Halachah does not follow R. Eliezer because he is Shmuti (from Beis Shamai).

Rebuttal #1 (Tosfos Bava Metzia 29a DH v'Havi): We cannot say that R. Akiva cannot hold like Rabah. The Gemara here said that both Tana'im hold like Rabah!

Rebuttal #2 (Rosh ibid.): Even though Rabah could say that the Tana'im argue about Shmuel's law, the Stam Gemara said otherwise. This is not a proof. Our entire Sugya assumes that the Tana'im argue when the security is worth less than the loan. This is why it did not say that they argue about Shmuel's law! Also, on 82a we strove to show that the Stam Mishnah is like Rabah. On 84a we established the Mishnah unlike R. Eliezer. According to Rav Yosef, we could have said that it is like everyone! Even though the Halachah follows Rabah, one who lends on a security is a Shomer Sachar. We follow R. Chananel, who says that Shmuel discusses one who specified that if he loses the security, he loses the loan, for all R. Chananel's words are Kabalah.

Korban Nesan'el (8): We follow R. Chananel's text. His rulings are not Kabalah; we often rule against him. However, in Nidah (1:5) the Rosh refused to rule leniently against R. Chananel, for his words are Kabalah.

Poskim

Shulchan Aruch (CM 267:16): As long as the Aveidah is with the finder, if it was stolen or lost he is liable, like a Shomer Sachar.

Rema: Some say that he is like a Shomer Chinam. The law is the same as we wrote regarding a security.

Shulchan Aruch (72:2): If one lent on a security, he is a Shomer Sachar.

Rema: Some say that he is only a Shomer Chinam if the security is worth more than the loan. If it was stolen or lost, he is exempt from paying the excess, but he does not get back the loan. Since it is a Safek, we do not force one to pay.

Beis Yosef (DH v'Sover): The Tur says that the Rosh rules like Rabah. R. Yerucham did not cite the Rosh, for he was unsure about his opinion. I say that since he did not give a clear ruling in his Pesakim, we rely on how he ruled in a Teshuvah, like Rav Yosef.

Rebuttal (Bach DH u'Mah): The Rosh clearly rules like Rabah in Shevuos! The Beis Yosef erred about his opinion.

Shach (23): If a lender lost the security and seized his debt from the borrower, we do not force him to return it. He can say 'I hold like the Poskim that a Shomer Aveidah is a Shomer Chinam, and regarding a lender, like those who do not say that it is as if he specified 'if I lose it, I lose from the debt corresponding to it.'

SMA (267:17): All hold that a lender is a Shomer Sachar on the value of the security up to the amount of the loan. The Rema equates the law of a Shomer Aveidah to that of a lender on the security's value above the amount of the loan.

Yam Shel Shlomo (Bava Kama 6:9): One may cover the blood (of a slaughtered bird or Chayah) with dirt that is Asur b'Hana'ah, for fulfilling Mitzvos is not considered Hana'ah. A Shofar or Lulav of Isur Hana'ah would be Kosher if no Shi'ur were required. We do not say that he benefits Perutah d'Rav Yosef by using Isurei Hana'ah for a Mitzvah! The Halachah follows Rabah. If a finder sold an Aveidah, he may use the money. This benefit makes him a Shomer Sachar. The Rambam rules that a Shomer Aveidah is a Shomer Sachar, so when he sells it and may use the money he is a borrower. This is wrong.

Rebuttal (Shach 14): The Rosh (it seems that it should say 'Ran') in Nedarim (33b DH Perutah) allows a Mudar Hana'ah to return an Aveidah because Perutah d'Rav Yosef is rare. Nevertheless, Perutah d'Rav Yosef suffices to make him a Shomer Sachar. Tosfos (Shevuos 44b DH v'Rav) asked about saving Perutah d'Rav Yosef while using Isurei Hana'ah for Mitzvos. He answered that Perutah d'Rav Yosef applies to an Aveidah or security because one must regularly spread it out (lest it get moldy), and an Oni might come then. One may blow a Shofar (or immerse in a Mikveh) of Isurei Hana'ah at a time when no Oni will come (so he gets no Hana'ah).

Conclusion (Shach): The Rema (72:2), SMA and all the Acharonim say that it is a Safek if the Halachah follows Rabah or Rav Yosef. If the one who lost the Aveidah seized the money from the finder, we do not force him to return it.

Or Some'ach (Schirus 10:1): We are not concerned for Perutah d'Rav Yosef regarding Isurei Hana'ah because physical benefit does not come from the forbidden object. It is like Mavri'ach Ari (averting a loss). Alternatively, we are concerned for benefit from Perutah d'Rav Yosef when the Mitzvah is due to the Isur. E.g. if a Mudar Hana'ah found an Aveidah of the one he may not benefit from, or if he lends to him only because he receives a security. These are unlike Mitzvos of Shofar and Lulav, which are obligatory on him in any case. His own obligation exempts him from Tzedakah.