“But if I am a shomer chinam and not liable for the necklace,” asked Mr. Braun, “why shouldn’t I be able to collect the full $5,000 of the loan?”
“Tosfos [s.v. Neima, Lo] explains, based on Shmuel’s opinion in the Gemara [ibid.], that there is an unstated understanding that if the collateral is lost, the borrower will not repay the loan,” replied Rabbi Dayan. “This is also the intention of the Mishnah that the lender is like a shomer sachar. When the collateral is equivalent to the loan, it cancels out; when it is worth more, the borrower is exempt from paying the loan but the lender does not have to pay the excess amount.” (SM”A 72:15; Shach 72:20)
“However,” concluded Rabbi Dayan, if the lender was negligent, he would be liable also for the excess amount. Conversely, if the collateral was lost through uncontrollable circumstances [oness] the lender is not liable at all and entitled to collect the full value of the loan, since he never accepted such responsibility on himself.” (See Pischei Choshen, Halva’ah 8:24)