1) MISUSE OF A RENTED OBJECT
QUESTION: The Gemara teaches that when a person rents a plow for use on mountainous terrain, and he uses it for that purpose and it breaks, the blame is placed on the workers and not on the renter, and thus he is exempt from liability. TOSFOS infers that if the renter misuses the plow (for example, he rents it for use in the valley, but he uses it for plowing on mountainous terrain), the renter is held accountable and the workers are exempt from liability. The reason why the workers are exempt is that they may claim that they accepted responsibility only if the plow was not being misused.
The RAMBAM (Hilchos Sechirus 4:1), however, rules that even in a case of misuse in which the renter is obligated to pay for the damages, the renter in turn may claim restitution from the workers.
According to the Rambam, why does the Gemara discuss the liability of the workers only in a case in which the plow was used for the purpose for which it was rented? According to the Rambam, the same liability (of the workers) applies in a case where the plow was misused!
ANSWER: The NESIVOS HA'MISHPAT (309:5) explains that even according to the Rambam, the workers are liable in a case of misuse only when the item broke as a result of a Peshi'ah, an act of negligence. The workers have the status of a Shomer Chinam because they are not paid in a case of misuse. In contrast, in the Gemara's case where the plow was used properly for the purpose for which it was rented, the workers have the status of a Shomer Sachar and are liable even for Aveidah. Therefore, the Gemara discusses a case in which the plow was used properly in order to teach a case in which the workers are liable even for Aveidah. (Y. MARCUS)
2) THE STATUS OF ONE WHO IS WATCHING A "MASHKON"
QUESTION: The Gemara teaches that a craftsman (Uman) is considered a Shomer Sachar because he receives benefit from holding the object upon which he worked -- the object serves as a security (Mashkon) for receiving his payment.
According to this reason, in every case of a loan with collateral the lender should have the status of a Shomer Sachar since he is holding the collateral and thus has a security for the repayment of the loan. The Gemara later (82a), however, gives different reasons for why a lender is considered a Shomer Sachar when he is holding an item of collateral. Why does the Gemara not give this reason?
ANSWER: TOSFOS answers that an ordinary lender is not considered a Shomer Sachar because he stands to gain nothing from the loan. Although by holding the collateral he is guaranteed to receive his money in return, he gains nothing as a result; he does not end up with anything more than he would have had in the event that he had not loaned any money in the first place. Hence, he is not considered a Shomer Sachar for the Mashkon. In contrast, the craftsman has much to gain -- the profits of his work -- and thus he is considered a Shomer Sachar for the Mashkon.
However, the RITVA (cited by the NIMUKEI YOSEF to Bava Kama, 39a of the pages of the Rif) records a ruling which seems to contradict the answer of Tosfos. The Ritva writes that one who inherits from his father a Mashkon that his father was holding for a loan that he had given is considered a Shomer Sachar for the Mashkon. Why is he considered a Shomer Sachar if he is not gaining anything from having the Mashkon? The MAHARSHAL (in Yam Shel Shlomo to Bava Kama 10:4) indeed writes that the Ritva argues with Tosfos.
The SHACH (CM 72:36), however, reconciles the ruling of the Ritva with the words of Tosfos. The Shach explains that there is a difference between a Mashkon taken at the time that the loan was given and a Mashkon taken after the loan was given. When Tosfos says that the lender gains nothing by having a Mashkon, he refers only to a Mashkon that was taken at the time of the loan. Tosfos agrees that a Mashkon taken later is considered a profit for the lender; since the loan has been given already, the taking of the Mashkon by the lender constitutes a clear benefit for him.
The Ritva, on the other hand, is discussing a case of a Mashkon that was taken after the loan was given. Therefore, he compares it to the Mashkon in the hands of a craftsman, and thus the lender is considered a Shomer Sachar.
The Shach adds that the son who inherits the Mashkon did not give a loan himself. Consequently, as far as he is concerned the Mashkon that he receives is pure profit (even if his father had received the Mashkon at the time the loan was given). (Y. MARCUS)