If one person says to another - you owe me 100 dinar, and the other says, I only owe you 50, I don't know ow about the other half - which means he can't swear, because he's not sure... he has to pay the entire amount. That case has a parallel in our mishnah... plus, if you borrow a cow and also the cow owner's services, the question is when is the borrower (or renter) liable? What if there are several stints of borrowing?