Bava Metzia 3B

והאי בכוליה בעי דלודי ליה. והאי דלא אודי ⁠– אשתמוטי הוא דקא מישתמט מיניה, סבר: עד דהוו לי זוזי ופרענא ליה, ואמר רחמנא: רמי שבועה עליה כי היכי דלודי ליה בכוליה,

And in order not to exhibit insolence, this person wants to admit to the creditor with regard to all of the debt, and this fact that he denies owing him in part is because he reasons: If I admit to him with regard to all of the debt, he will lodge a claim against me with regard to all of it, and right now I do not have the money to pay. He was evading his creditor, and thought: I will continue doing so until I have money, and then I will pay him all of it. This rationalization enables one to falsely deny part of a claim. And therefore, God states: Impose an oath on him, in order to ensure that he will admit to him with regard to all of the debt.

אבל העדאת עדים, דליכא למימר הכי ⁠– אימא לא. קא משמע לן קל וחומר.

But in a case where the testimony of witnesses renders him liable to pay part of the debt, as one cannot say this explanation since this logic applies only when it is the debtor admitting to part of the claim, say that he has no intention to repay the debt at all, and he is completely dishonest and therefore his oath is worthless. Therefore, the baraita teaches us that the defendant’s obligation to take an oath is derived by means of an a fortiori inference.

ומאי קל וחומר? ומה פיו שאין מחייבו ממון ⁠– מחייבו שבועה, עדים שמחייבין אותו ממון ⁠– אינו דין שמחייבין אותו שבועה?

The Talmud explains: And what is the explanation of this a fortiori inference? It should be explained as follows: If admitting to part of a claim by his own mouth, which does not render him liable to pay the money he admitted to owing, nevertheless obligates him to take an oath, with regard to the testimony of witnesses, which does render him liable to pay money, is it not logical that it should obligate him to take an oath with regard to the remaining sum?