An unpaid custodian may stipulate to be exempt from an oath, and a borrower may stipulate to be exempt from paying.
Whoever
makes a stipulation contrary to what is written in the Torah, his
stipulation is void. Any stipulation after the action is void. For
example, “This item is yours on condition that you walk a mile” - the
items is his and he does not have to walk. Any stipulation that is
impossible to fulfill is void. For example, “This is your Get on
condition that you cross the sea on foot” - the condition is void, the
Get is valid, and the woman is divorced.
If one borrows a cow but hires the owner along with it, he is not liable if the cow dies.