Bava Metzia 3 - Borrower Admits Loan In Part
If one says, "You owe me $100," and the borrower admits to owing $50, the borrower has to repeat his statement under oath , and pay $50.
There is a presumption that people are not brazen to deny a loan altogether, since the creditor did them a favor. Likely, the borrower would like to repay, but doesn't have the complete amount, so he admits to a part, thinking to repay the rest later. Thus the Torah imposed an oath to force him to tell the truth.
Likewise, if witnesses testified that the borrower owes part of the loan, he has to take an oath for the rest.