Baba Metzia 7:10-11

Baba Metzia 7:10

If an animal dies naturally, it is considered to be beyond the watchman’s control. If he mistreated the animal and it dies, it is not beyond his control. If an animal climbed to the top of a cliff and fell, it is considered beyond the guardian’s control. If he led it to the top of cliff and it fell and died, it is not considered beyond his control. An unpaid watchman may make a condition to exempt himself from having to take an oath, a borrower may make a condition to exempt himself from paying, and a paid watchman and a renter may make a condition to exempt themselves from taking an oath and from paying.

Baba Metzia 7:11

If a person makes a condition that goes against what is written in the Torah, the condition is invalid. If a condition starts by mentioning the action, the condition is invalid (i.e., the form of a condition must be “if X, then Y” and not “Y, if X”). If a condition can ultimately be fulfilled (i.e., it is not impossible) and is made at the beginning, then it is valid.