Bava Metzia, Chapter 8 Mishnah 3 - the last of the mishnayos dealing with the laws of watchmen (shomrim). The foundation of the Mishnah is simple: A borrower is liable for unavoidable accidents (oness), but this liability only applies to him from the moment the borrowed item enters his domain. From here the Mishnah seeks to clarify the rule of an intermediary, a person who transfers the item from the lender to the borrower: At what stage does the borrower accept responsibility and become liable even for unavoidable accidents?
The short answer:
The item becomes borrowed from the time it reaches the hand and domain of the borrower, or the hands of a person he appointed and authorized to be his agent. As long as the borrower did not authorize an agent, there is no difference regarding the identity of the recipient - whether it is his son, his slave, or his regular agent. If the lender handed the item to one of these, it is not yet considered a borrowed item for which the borrower is liable for unavoidable accidents, until it reaches his hand or the hand of his authorized agent. This is the point of our Mishnah.
The text of the Mishnah - the exempt case:
"Ha'sho'el es haporah v'shilchah lo b'yad bno, b'yad avdo, b'yad shlucho, o b'yad bno, b'yad avdo, b'yad shlucho shel sho'el, umeisah - patur" - Meaning, the owner of the cow who is lending it sends it via one of the following intermediaries:
"B'yad bno, b'yad avdo, b'yad shlucho" - The son, slave, or agent of the lender.
"O b'yad bno, b'yad avdo, b'yad shlucho shel sho'el" - Even the son, slave, or agent of the borrower, as long as they were not authorized to act on his behalf.
"Umeisah - patur" - If the cow died while in the possession of that intermediary, the borrower is exempt from unavoidable accidents, since he has not yet received it into his domain and has not become a borrower who is liable for such responsibility.
The liable case - when the agent was appointed:
"Amar lo hasho'el: shilchah li b'yad bni, b'yad avdi, b'yad shluchi, o b'yad bincha, b'yad avd'cha, b'yad shluch'cha" - In all of these cases, the borrower is liable from the moment the cow reaches the hands of the intermediary, since he himself requested that it be sent in this way.
"O she'amar lo hamash'il: hareni meshalchah lecha b'yad bni, b'yad avdi, b'yad shluchi, o b'yad bincha, b'yad avd'cha, b'yad shluch'cha, v'amar lo hasho'el: shlach, v'shilchah umeisah - chayav" - Even when the initiative came from the lender, and it makes no difference who the intermediary is, since the borrower replied "send it" and agreed to the method of delivery, and the cow died while still in the hands of the intermediary - he is already considered a borrower, is liable for unavoidable accidents, and must pay the value of the cow that died.
A qualification regarding the slave:
A Canaanite slave is considered a legal extension of his master. Therefore, the case where the lender's slave serves as an intermediary on behalf of the borrower, and was even authorized to do so, must be dealing with a Hebrew slave, who can act as the agent of the recipient. But regarding a Canaanite slave, even if he was authorized, the item is still considered to be resting in the hands of the lender until it physically passes into the hands of the borrower.
"Vechen besha'ah shemachazirah" - And similarly when he returns it:
The same rule applies even when returning the animal: the cow remains in the borrower's domain and under his responsibility until he transfers it to the lender's domain - whether directly into his hand and actual domain, or to an agent whom the lender authorized to act on his behalf. This concludes the Mishnah.
One last point - the definition of the borrowing period:
A person is only considered a borrower during the time he is permitted to use the object. For example, someone who borrowed a cow to plow his field on Tuesday, and received the lender's permission:
If the cow was handed over to him earlier, on Monday, when he is not yet permitted to use it, since the borrowing is only for Tuesday.
If, upon finishing his work, he did not return it immediately, and he is not required to return it until Wednesday, and he indeed only gets around to returning it then.
In both cases, since he is not permitted to use it - because the borrowing period has not yet begun or has already ended - he does not bear the responsibility of a borrower, but rather has the status of a paid watchman only, who is exempt from unavoidable accidents. Therefore, if an unavoidable accident occurred after the borrowing period ended or before it began, this person who would be considered a borrower at another point in time - is exempt.