פירוש על בבא מציעא 8:3

ביד בנו – The lender sent it to the borrower by the hand of his son or his servant or the agent of the lender.

If one borrowed a cow and the owner sent it by the hand of his son or his slave or his agent, or by the hand of the borrower’s son or slave or agent and it died [on the way], the borrower is not liable.
But if the borrower said, “Send it to me by the hand of my son or my slave or my agent, or by the hand of your son or your slave or your agent”, or if the lender said, “I am sending it to you by the hand of my son or my slave or my agent”, or “by the hand of your son or your slave or your agent”, and the borrower said, “Send it”, and he sent it and it died [on the way], the borrower is liable.
So, too, when the cow is returned.
Mishnah three deals with a lender who uses an agent to deliver the object to be borrowed to the borrower.
Mishnah four deals with disputes in sales where it is either unknown whether the cow being sold gave birth before it was sold (in which case the calf belongs to the seller) or after it was sold (in which case the calf belongs to the buyer). Similarly this mishnah deals with a dispute between a buyer and a seller over which field or slave was sold, the larger or the smaller one.
The issue in this mishnah is the liability of the borrower during the time when the borrowed animal is being delivered from the lender to the borrower. If the lender sent it with a third party, be it his own son, slave or agent or even the borrower’s son, slave or agent, without being asked to do so by the borrower then the borrower is not liable for the animal until it reaches him. The borrower can say to the lender that he did not agree to accept liability until the animal reached his hand.
If, however, the borrower requested that the lender send the object with someone else or the lender told him that he was going to do so and the borrower agreed, then the borrower is liable for the object as soon as the animal leaves the hand of the lender. Since the borrower agreed that the animal could be delivered by a third party, it is as if he has accepted responsibility immediately.
All that was stated in the previous part of the mishnah is also true at the time when the animal is being returned. If the borrower sends the animal back through a third party without being asked he is responsible for the animal until it reaches the lender. If, however, the lender told him to return it through a third party or the borrower told him that he was returning it through a third party and the lender agreed, from the moment it leaves his hands the borrower is no longer liable for the animal.

או ביד בנו ועבדו ושלוחו של שואל – he (i.e. the borrower) is exempt if it (i.e. the animal) died on the road [to him/her]. But that agent of the borrower, one can establish it (see Bava Kamma 104a – also Shevuot 46b) – regarding his hired laborer or his client who lives in his home (may they take the oath)? But he did not make this person an agent before witnesses, for if witnesses had made the person the agent, the borrower would be obligated for his (i.e., the agent’s) unavoidable accident when the lender transferred it (i.e., the animal) to him. But there are those who say that even if he (i.e., the borrower) made the person his agent before witnesses, he is not liable through his hand for unavoidable accidents. And this is what he says to him – that a person is [deemed] believable if he wishes to send it in his hands, he sends it.

אמר לו השואל שלחה לי וכו' – in the hand of your servant, he (i.e., the borrower) is liable. This “your servant’ is speaking about a Hebrew slave, for it were a Canaanite slave, the hand of the slave is like the hand of the master, and it would be like he didn’t depart from the domain of the lender, and it would be like the lender himself walked to him, and the borrower would be exempt, if he suffered an unavoidable accident on the way.

וכן בשעה שמחזירה – If the borrower sent it in the hand of his son, or his servant or his gent, or in the hand of the son, slave or agent of the lender, it did not leave the domain of the the borrower juntil it comes to the hand of the lender, and if he suffers an unavoidable accident on the way, he is liable. If the lender said to him (i.e., the borrower): “Send it to me,” or the borrower said, “Behold I am sending it, et.” And the lender said to him, “Send it,” and he sent it (i.e., the animal) and it suffered an unavoidable accident on the way, he is exempt [from having to make any payments). And our Mishnah specifically when he returns it amidst the days of his borrowing it (i.e., the animal) when he is liable for unavoidable accidents. But if he returned it after the days of his borrowing, the law of a paid bailee applies [since he derived benefit from it] and not the law of a borrower, and if he sent it by the hand of his son, or his servant ,or his agent, whether it was his or it belonged to the lender, and it suffered an unavoidable accident, he is exempt [from payment].