Shevuot (Oaths) 41 - A Borrower Would Not Outright Deny a Loan
If the plaintiff says, "You have a maneh ($5,000) of mine in your possession," and the defendant responds, "You do not have anything of yours in my possession," - the defendant is free from an oath, since there is no admission of partial liability.
The Torah assumes that no person would be so impudent as to deny a loan outright. After all, the lender did him a favor by loaning money interest-free (and other types of loans are anyway forbidden). When the borrower partially admits a loan, but partially denies it, this is just because he does not have all the money and plans to pay the balance later on. This is not right, however, and the Torah imposes an oath on him. But we cannot assume this in the case of complete denial.
Later, however, this situation has changed, and the Sages established an "Oath of Forgetfulness" even for cases of complete denial. However, this Rabbinical oath is different in that it can be switched: if the defendant does not want to take an oath, the plaintiff can take an oath and collect the money.
Art: Leopold Loeffler - Forgotten Speech