Mishnayos Shevuos Perek 8 Mishnah 1

There are four types of bailees, to whom different halakhot apply. They are as follows: An unpaid bailee, who receives no compensation for safeguarding the item in his care; and a borrower, who receives an item on loan for his own use without paying a rental fee; a paid bailee, who is paid a salary for safeguarding the deposited item; and a renter, i.e., one who pays a fee for the use of an item or animal. In the event that one of these bailees is unable to return the deposited item to its owner, the halakha with regard to liability is dependent upon what happened to the item, and upon the type of bailee: An unpaid bailee takes an oath attesting to the fact that he was not negligent with the care of the item and is then exempt from liability for everything, meaning for all types of damage, whether the item was lost, stolen, damaged, or if the animal died. Conversely, a borrower does not have the option of taking an oath, and pays for everything, whether the item was stolen, lost, damaged, or the animal died, even if it was by unavoidable accident. The halakhot of a paid bailee and a renter are the same: They take an oath concerning an injured animal, and concerning a captured one, and concerning a dead animal, attesting to the fact that these mishaps were not caused by negligence, but rather by unavoidable accident, and then are exempt from liability. But they must pay if the deposit cannot be returned due to loss or theft, even if these were not caused by negligence.

אַרְבָּעָה שׁוֹמְרִין הֵן, שׁוֹמֵר חִנָּם, וְהַשּׁוֹאֵל, נוֹשֵׂא שָׂכָר, וְהַשּׂוֹכֵר. שׁוֹמֵר חִנָּם נִשְׁבָּע עַל הַכֹּל. וְהַשּׁוֹאֵל מְשַׁלֵּם אֶת הַכֹּל. נוֹשֵׂא שָׂכָר וְהַשּׂוֹכֵר נִשְׁבָּעִין עַל הַשְּׁבוּרָה וְעַל הַשְּׁבוּיָה וְעַל הַמֵּתָה, וּמְשַׁלְּמִים אֶת הָאֲבֵדָה וְאֶת הַגְּנֵבָה: