Ketubot 18B

ובכולה בעי דלודי ליה. והאי דלא אודי ליה ⁠— כי היכי דלישתמיט ליה, וסבר: עד דהוה לי זוזי ופרענא ליה. ורחמנא אמר: רמי שבועה עליה, כי היכי דלודי ליה בכוליה.

And, as a result, he would have liked to admit to him that he owes him the entire loan. And the reason that he did not admit to him that he owes him the entire loan is so that he may temporarily avoid paying him. And he rationalizes doing so, saying to himself: I am avoiding him only until the time that I have money, and then I will repay him. Due to the concern that the partial admission is motivated by that rationalization and the claim of the lender is true, God says: Impose an oath upon him so that he will admit that he owes him the entire loan.

רבי אליעזר בן יעקב סבר: לא שנא בו ולא שנא בבנו, אינו מעיז, והלכך לאו משיב אבידה הוי. ורבנן סברי: בו הוא דאינו מעיז, אבל בבנו ⁠— מעיז. ומדלא העיז ⁠— משיב אבידה הוי.

R' Eliezer ben Ya’akov maintains: It is no different with regard to the creditor himself, and it is no different with regard to his son. The debtor would not be so insolent as to deny the debt. And therefore, he is not considered as one returning a lost article on his own initiative. Rather, he is considered as one who partially admits his debt in response to a claim, and is therefore required to take an oath. However, the Rabbis maintain: In the presence of the creditor one would not be insolent, but in the presence of his son, who did not lend him the money, he would be insolent and deny the claim entirely. Since he had the option of completely denying the loan and opted to admit to part of the claim, he is considered as one returning a lost article and his claim is accepted without an oath.