Gemara

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

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

Question (Rav Yosef bar Chama - Beraisa): If Hekdesh hired Reuven to guard crops, for the week, he is paid (even) for Shabbos, therefore, he is responsible for (losses on) Shabbos.

Answer (Rav Sheshes): He made a Kinyan obligating himself to pay if there will be a loss.

Bava Kama 62a (Rava): If Reuven gave to Leah a gold coin, and told her 'be careful with it, for it is silver', and she damaged it, she pays its full value, for she had no right to damage it. If she was negligent and it was damaged, she pays for silver. She can say 'I accepted to guard only a silver coin.'

Rishonim

Rif: If he did a Kinyan to accept liability, he pays.

Rambam (Hilchos Sechirus 2:1): The three laws of the four Shomerim apply only to Metaltelim of a Yisrael commoner. It says "money or Kelim or any animal." This excludes land, slaves (which are equated to land), and documents, which have no intrinsic value. "When one will give to his fellowman" excludes Hekdesh and property of Nochrim. From here, Chachamim said that a Shomer Chinam does not swear about land, slaves or documents, and a Shomer Sachar or renter does not pay. If he acquired, he is liable.

Rambam (3): It seems to me that if the Shomer was negligent with slaves or similar matters, he must pay.

Ritva (Teshuvah 199, cited in Beis Yosef CM 66 DH Kosav ha'Ritva): The Rambam holds that the Torah excluded documents only from what is written in the Parshah, i.e. Ones, theft or loss. One is liable for negligence, due to Mazik. If there is no proof of negligence, he need not swear that he was not negligent or Shole'ach Yad, and that it is not in his Reshus, for the Mishnah says absolutely that he does not swear. All my Rebbeyim hold like the Rif, who exempts. Negligence is only Gerama, so it is not liable b'Yedei Adam like a Mazik. The Torah obligates a Shomer for negligence, because the owner relied on him. If one says that he returned documents, he should be exempt even from Shevu'as Heses, due to a Migo that he could say that they were lost. However, later Chachamim enacted Heses, even though the Torah exempts. Migo does not exempt from Heses more than the Torah! Some of my Rebbeyim exempt documents even from Heses.

Rambam (ibid): My Rebbeyim ruled that if one gave a vineyard to a Shomer, whether a sharecropper or a Shomer Chinam, and stipulated that he dig or prune o r Ye'avek (put dust on the top) from his own, and he was negligent and did not do so, he is liable like one who overtly caused a loss; he is liable in any case.

Ra'avad: A Mishnah (Bava Metzia 104a) teaches this! If I will leave it fallow and not work it, I will pay. Chachamim made this like a Tenai Beis Din. If a worker ruined his task, obviously he is like a Mazik! We say that a planter, slaughterer or bloodletter is like one who was warned (that if he ruins his task, he is fired - Bava Metzia 109a).

Rambam (Hilchos To'en 5:6): If Shimon says 'I deposited a document with you. It contained a proof of 10 Dinarim (that are owed to me)', and Reuven said 'nothing ever happened', he swears Heses, and he is exempt. If Reuven said 'yes, you deposited a document with me. It was lost', he is exempt even from Heses. Even if he would admit that he was negligent and lost it, he would be exempt, like I said in Hilchos Chovel.

Magid Mishneh: In the Seifa, Reuven does not believe that Shimon lost due to loss of the document. He need not swear, for Shimon does not have a Vadai claim.

Rosh (4:21): The Ra'avad and Ramban exempt only from swearing, but if he admits or witnesses testify that he was negligent, and it was stolen or lost, he is liable. Teshuvas ha'Rif exempts a Shomer who was negligent witgh documents. This seems correct. Even though a Shomer Sachar does not pay, he is not paid until he swears that he guarded properly. If he acquired, he is liable.

Beis Yosef (CM 66 DH u'Mihu): The Magid Mishneh (Hilchos Sechirus 2:2) proves from Bava Metzia 58a that he does not lose his wages if Ones occurred.

Rosh (Shevuos 6:24): Also a borrower is exempt for these things. The Mishnah exempts a Shomer Chinam and Shomer Sachar, so it did not need to teach that a borrower is exempt. Also, it taught only things that apply to Hekdesh.

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. However, he loses his wages until he swears that he guarded properly.

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. However, he is liable for overt damage, e.g. he cast them into a river.

Beis Yosef (ibid.): The Rosh (39:2) says that even the Rif exempts only theft or loss due to negligence, but not if he cast them into a river. For this reason, he obligated a scribe who wrote a document for Shimon and gave it to his father. This is like overt destruction, and he is liable, unless the father came to the scribe as the son's Shali'ach, and it is known that the son trusts the father with all his property. In such a case, the scribe was not negligent.

Shach (127): The Rema (301:1) obligates even if he merely made the father a Shomer. This is wrong. This is not worse than negligence, for which the Rosh exempts! If a Shomer gave to a Shomer, the Rosh (Bava Metzia 3:6) does not call this negligence. He obligates the scribe only if he totally gave it to the father.

Gra (158): We learn from Bava Kama 62a. Even one who is exempt from obligations of a Shomer (for gold, for she was asked to guard silver) is liable for overt damage.

Shulchan Aruch (ibid): If he acquired to be liable like a Shomer, he is liable.

Beis Yosef (DH v'Da): The Gemara discusses only a Shomer Sachar obligating himself to pay. A Shomer Chinam cannot obligate himself to swear (mid'Oraisa)! Why did the Tur omit this?

Rebuttal (Darchei Moshe 14): There is no reason to distinguish. Even a Shomer Chinam can obligate himself to swear. Even though the Gemara taught this only regarding a Shomer Sachar, the Rosh says that a borrower is exempt from liability of Shemirah for land, even though the Gemara taught only about a Shomer Chinam and Shomer Sachar! The Tur did not need to teach this, for it is obvious, just like a Shomer Chinam can stipulate to be liable like a Shomer Sachar (291:28).

Defense (Shach 128): A Kinyan to pay takes effect. A Kinyan to obligate swearing is like a verbal Kinyan, which is invalid (Bava Basra 3a). The Rema himself (157:4) says that a verbal Kinyan is invalid!

SMA (90): The Tur and Shulchan Aruch (291:28, 305:4) say that a Shomer Chinam can obligate himself to pay like a Shomer Sachar or borrower through words without a Kinyan! It is because he is a Shomer. Here, one who guards land is not a Shomer.

Shach (129): A Kinyan is needed to obligate something that the Torah never obligates. There, it is as if he says 'I am like a borrower.'