Commento su Bava Metzia 3:1

המפקיד. ולא רצה לישבע – the oath of the bailees, for he was able to make himself exempt if he took an oath that he did was not negligent with it and did not make illegitimate use of it.

If a man left an animal or utensils in his fellow’s keeping and they were stolen or lost, and his fellow made restitution and did not want to take an oath – for they have taught: an unpaid guardian may take an oath and be exempt from liability – if the thief is found he must make twofold restitution, and if he slaughtered or sold the [sheep or ox] he must make four of fivefold restitution. To whom does he pay? He with whom the money was deposited.
If his fellow (the unpaid takes an oath and does not want to make restitution, if the thief is found he must make twofold restitution, and if he slaughtered or sold the [sheep or ox] he must make four of fivefold restitution. To whom does he pay? To the owner of the property deposited.
According to the Mishnah there are four different types of guardians or people who have responsibility to watch over other people’s objects. Each guardian has a different level of liability should the object be lost, damaged or die (if it was an animal). The Torah discusses guardians in Exodus 22:6-14.
1) The borrower. He is responsible to return the object at its original value to the owner no matter what happens to it. For instance should a person borrow a cow and the cow die even a natural death he would be obligated to give the owner the value of the live cow.
2) The renter. If the animal should be damaged or taken captive or die while he was renting it he may take an oath that he was not negligent with the animal and he is then exempt from paying back the value of the animal. If the animal should be stolen or lost he must repay the value of the animal.
3) The paid guardian, i.e. someone who is paid to watch the animal or object. His liability is the same as the renter’s.
4) The unpaid guardian. No matter what happens to the animal or object he may take an oath that he was not negligent in its care and then be exempt from paying.
As you will notice, the greater the benefit the greater the liability. A borrower, who uses the object and does not pay for the use only benefits. Therefore his liability is greatest. An unpaid guardian, who is not allowed to use the object, does not benefit at all. Therefore his liability is minimal. A renter gets to use the object but he pays for it. A paid guardian may not use the object but he gets paid for watching over it. Therefore renters and unpaid guardians have middle level liability.
These laws are a necessary background to many of the rules in our chapter and they will be dealt with at greater length in chapter eight.
Our mishnah deals with an object that is stolen or lost while under the watch of an unpaid guardian.
The scenario in the first half of the mishnah is as follows: a man leaves an animal or utensils with his friend who is an unpaid guardian. The animal or utensils are stolen by a thief. When the owner asks for them in return the unpaid guardian elects to repay the value instead of taking an oath. Assumedly he preferred to pay the value rather than take an oath lest the oath be a false oath. When the thief is found the thief must make twofold restitution if the animal was still alive and in his possession or fourfold or fivefold restitution if he had sold or slaughtered the animal (see chapter seven of Bava Kamma). The question our mishnah asks is who receives the payment. After all someone is going to benefit by receiving up to five times the value of the animal. In this scenario the guardian receives the money and the extra money as well. Since he already repaid the owner, it is as if he acquired the animal and all subsequent benefits that would come from the animal.
The difference in the second half of the mishnah is that the unpaid guardian took an oath and did not repay the owner. In such a case if the thief were to be found he would repay the value plus the penalty to the original owner.

למי שהפקדון שלו – for since he paid, he has acquired all of its indemnities. And even if he did not pay but rather since he said in the Jewish court that he would pay, he has acquired all of the indemnities. There is no difference made between double indemnity or four/five times the indemnity.