Gemara

56b (Mishnah): A Shomer Chinam does not swear about slaves, documents, land, and Hekdesh, and a Shomer Sachar does not pay,

57b (Beraisa): "Ki Yiten Ish El Re'ehu" is a Kelal. "Chamor Oh Shor Oh Seh" are Pratim. "V'Chol Behemah Lishmor" is another Kelal. The Kelal u'Frat u'Chlal teaches everything similar to the Pratim, something movable with intrinsic value. This excludes land, slaves and documents. "Re'ehu" excludes Hekdesh.

58a (Mishnah): if people of a city made Sheluchim to take their (half-Shekalim to the Mikdash, and they were stolen or lost before Terumah was taken (some of the Shekalim collected were taken to buy the communal Korbanos), the Sheluchim swear to the people of the city (and they must send more Shekalim).

(Shmuel): The case is, the Sheluchim were hired. They must swear in order to receive their wages.

Question: A Shomer Sachar is liable for theft or loss! Granted, regarding Hekdesh they are exempt, but they should not be paid!

Answer #1 (Rabah): It was Ones. Armed robbers stole them, or they were lost on a boat that sank.

Answer #2 (R. Elazar): This oath is mid'Rabanan, lest people treat Hekdesh lightly.

Contradiction (Rav Yosef bar Chama - Beraisa): If Hekdesh hired Reuven to guard crops, he is not paid for Shabbos, therefore, he is not liable for what happens on Shabbos. If he was hired for the week, he is paid (even) for Shabbos, therefore, he is responsible for (losses on) Shabbos.

Answer #1 (Rabah): No, it means that if a loss occurs on Shabbos, he is not paid for Shabbos.

Objection (and Answer #2 - Rav Yosef): The Reisha cannot refer to losing his wages, for he is not paid for Shabbos! Rather, Rav Sheshes taught that he made an acquisition obligating himself to pay if there will be a loss.

Bava Kama 93a: Rav Yosef deposited a wallet with Tzedakah money with Ploni. He was negligent, and thieves took it. Rav Yosef obligated him to pay.

Question (Abaye - Beraisa): "To guard", not to distribute to Aniyim.

Answer (Rav Yosef): (That applies in most places, where we do not know who was destined to receive the money, so no Oni can claim it.) Here, each Oni gets a fixed amount from Tzedakah. Ploni was like a guardian for every Oni.

Rishonim

Rambam (Hilchos Sechirus 2:3): It seems to me that if a Shomer was negligent with slaves or similar matters, he must pay. He is exempt for slaves, land and documents only for theft, loss, death and similar matters. A Shomer Chinam for Metaltelim must swear if they were lost or stolen. He need not swear about sld9. Similarly, a Shomer Sachar pays for theft or loss of Metaltelim, but he is exempt for these. If he was negligent he pays, for he is like a Mazik (damager). Damaging land is holds like damaging Metaltelim.

Rebuttal (Ra'avad): If so, a Shomer Chinam should swear. Perhaps he was negligent, and he is liable! This is unlike Modeh b'Miktzas, who would pay if he admitted, but even so he does not swear. A Kelal u'Frat teaches about a Shomer Chinam, and one teaches about a Shomer Sachar. Just like the Torah exempted a Shomer Sachar from his primary liability to pay, not just from swearing, the same applies to a Shomer Chinam. One who was negligent is not a Mazik. If he were, negligence b'Ba'alim would be liable.

Defense (Shach CM 66:126): Modeh b'Miktzas and one witness refute the Ra'avad (even though perhaps he owes, a Gezeras ha'Kasuv exempts from swearing). The Ra'avad said that just like the Torah exempted a Shomer Sachar from his primary liability to pay, not just from swearing, the same applies to a Shomer Chinam. I say that the Torah did not Mechadesh that even a Shomer Chinam pays for negligence. We know from reasoning that he is a Mazik. (Therefore, he is liable even if he is not considered a Shomer, e.g. for land.) The Rambam did not mean that it is literally Mazik, rather, it is like Mazik, that we obligate due to reasoning. (Therefore, it is exempt b'Ba'alim.)

Rosh (4:21): It seems that we exempt only from swearing, but if he admits or witnesses testify that he was negligent, and it was stolen or lost, he is liable. The Ra'avad and Ramban say so. The Rif wrote in a Teshuvah that if one deposited documents and the Shomer was negligent, he is exempt. This seems correct. Since the Torah excluded them from Parshas Shomerim, we can obligate only for Mazik (one who damaged). Negligence that led to loss is not Mazik, for a Shomer b'Ba'alim (the depositer was working for the Shomer) is exempt for negligence. If negligence were like Mazik, he would be liable! Rather, he is not like a Mazik, since he did not overtly damage it. It was lost due to his negligence. The Torah obligates a Shomer for this, therefore, b'Ba'alim he is exempt.

Rosh (Shevuos 6:24): The Mishnah said 'a Shomer Chinam does not swear about them.' It did not say that he does not pay for them, for the Parshah of Shomer Chinam did not discuss payment. It only discusses Shevu'ah. The Mishnah mentions only what the Torah mentioned regarding these Shomerim. E.g. the Torah says that a Shomer Sachar pays for theft or loss, so the Mishnah says that he does not pay. Likewise, a Shomer Chinam is exempt (for land, slaves and documents) for his liability, i.e. negligence. Bava Kama 93a proves this. Abaye wanted to exempt the Shomer due to "to guard", not to distribute to Aniyim. This shows that one need not pay for what is excluded from the Parshah. The exclusion is not only for swearing. Even a Shomer Sachar is exempt for negligence, for the Shomer of Tzedakah was exempt from giving Tzedakah (while caring for the money), and Rav Yosef holds that this makes him a Shomer Sachar. The Ramban and Ra'avad obligate for negligence.

Bach (CM 66:39 DH Kosav): This is a textual mistake. It should say 'the Rambam and Ramah obligate.'

Shach (ibid.): There is no mistake. There were two Ra'avad's. Alternatively, in his Perush, the Ra'avad retracted from what he wrote in his comments on the Rambam. The Ramban in his treatise on Garmi brought both opinions and did not decide.

Tosfos (57b DH Shomer): The Gemara asked why one hired to guard Hekdesh is liable, and did not answer that he was negligent. This shows that (for Hekdesh, land...) even a Shomer Sachar is exempt for negligence.

Poskim

Shulchan Aruch (CM 66:40): If one was a Shomer Sachar on documents and they were lost or stolen, even through negligence, he is exempt.

Beis Yosef (DH v'Chosav): The Rosh (94:5) says that we rely on the clear proofs of the Rif, Ra'avad and Ri, and not on the Rambam, who wrote (as if he heard this Halachah through) prophecy, without any proof.

Shach (126): The Shulchan Aruch rules like this also in Siman 95:1 and 301:2. However, the Gemara supports the Rambam. We do not follow the majority when the Gemara supports the minority, especially when the majority relied on invalid proofs. In Bava Kama, '"to guard", not to distribute to Aniyim' teaches that he is not called a Shomer. Therefore, he is like a Stam person, and surely he is exempt for negligence. Tosfos brought a proof from liability of a Shomer Sachar for Hekdesh. The Gemara did not answer that he was negligent. I say that this is because 'he is liable for Shabbos' connotes liability like a regular Shomer, not only for negligence. Also, the Reisha said 'he is not paid for Shabbos, therefore, he is not liable for Shabbos.' (The Rambam holds that) this cannot refer to negligence, for even a Shomer Chinam is liable for negligence. One could reject this and say that (really, we discuss negligence;) since he is not paid for Shabbos, he is not a Shomer at all for Shabbos! However, the Gemara could understand that since he accepted Stam, and it is in his Reshus on Shabbos, he accepted Shemirah even for Shabbos. The Mishnah did not simply say that Shemirah does not apply to land. It elaborated to say that a Shomer Chinam does not swear about them... to teach that he is exempt only from swearing, but if he admits that he was negligent, he pays. In Shevuos (42b) it says that one does not swear. This implies that only Shevu'ah is excluded. R. Elazar said 'this oath (about lost Shekalim) is a stringency mid'Rabanan, lest people treat Hekdesh lightly. Surely, only the oath is mid'Rabanan. We would not obligate them to pay due to this concern if the Torah exempts them! Also, he said 'this oath' to teach that only the oath is mid'Rabanan but not liability to pay. Similar, Rashi (56a DH Eino) says 'the Torah did not obligate swearing that he was not negligent.'

Question: What is the source to exempt negligence b'Ba'alim, and to obligate it for land?

Answer (Shach): A Kelal u'Frat exempts slaves, documents and land from laws of Shemirah. We rely on reasoning, to obligate only for negligence. "Ba'alav Ein Imo" exempts Shemirah b'Ba'alim even for negligence.

Rema: All the more so, a Shomer Chinam is exempt for negligence.

Shulchan Aruch (ibid): Some obligate for negligence.

Rema: The first opinion is primary. He is exempt even for negligence.