The fourth category of watchman is the borrower, one who takes an object for his own benefit (as opposed to as a favor to the owner) without any money changing hands. The Torah tells us in such a case that if the object breaks or the animal dies in the owner’s absence, then the borrower must pay for it. (If the owner is present, the borrower doesn’t have to pay.)
A borrower is different from a renter, who only swears that the damage is not his fault, because the renter paid for the use of the object and the money covers the risk of accidental damage. A borrower enjoys pure benefit at no expense, therefore he has greater responsibility.
So, now that we have seen all four categories of watchmen, let’s compare the limits of their liabilities. Basically, the unpaid watchman only pays for damage caused by his own negligence. A renter or paid custodian is also responsible for loss or theft. A borrower is additionally liable in cases of accidental damage.
The borrower is exempt from paying if the owner is there because the owner will keep an eye on his own property and he can see what occurs to it.
The exception to the borrower’s liability is if the object breaks or the animal dies in the course of its normal work. If a corkscrew breaks while opening a bottle, that’s not the borrower’s fault. If it broke while the borrower was using it to drill holes in his wall… well, who told him to use a corkscrew as a drill? That’s certainly his fault!
This mitzvah applies to the judges, who are obligated to try such cases. In the Talmud, it is discussed in the tractates of Baba Metzia (chapter 8), and Shevuos (also chapter 8). In the Shulchan Aruch, it is found in Choshen Mishpat 293, 303 and 306. It is #244 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.