Commentary for Bava Metzia 67:20

3. A Borrower that said I will pay etc. By the other's (the paid and unpaid watchman) its not enough for him (Rav papa) to say the language that (the watchmen said) behold I will be paying, for he wants to teach us the (Halchik) rule (that the caretaker buys the right to the double etc) even if he (the watchman) said I was negligent.