Mechaber: In response to a claim for one maneh (the equivalent of 100 dinarim), the defendant admits he owes fifty dinarim but denies he owes the other fifty. In this situation, the Torah requires him to swear that he owes only fifty. This oath is known as the Modeh Bemiktzat oath of partial admission. After taking this oath, he must pay the fifty that he acknowledges. If, however, the defendant has one supporting witness who testifies that he indeed repaid fifty, he is, according to some halachic opinions, excused from taking an oath regarding the balance, just as he would be had two witnesses supported him. According to other opinions, however, he is not exempt.
Ner Eyal : The reason for the imposition of the Modeh Bemiktzat oath of partial admission is to discourage a borrower who knows he owes the balance from denying it until he can afford to pay. The serious and irrevocable consequences of taking a false oath would deter him from using such delay tactics.
The reason one witness supporting the defendant excuses him from taking the Modeh Bemiktzat oath of partial admission is based on the same logic that obliges him to take the oath when one witness testifies against him. The basic rule is Hamotzi Meichaveiro Alav Hare’ayah, which means the plaintiff must prove his case before he can extract money from the defendant. If he cannot, the defendant wins without having to take a Torah oath.
This basic rule is displaced, however, when one witness testifies against the defendant. The testimony of one witness is not enough to win the case for the plaintiff. In order to prevail, a claim must be supported by two witnesses. But the testimony of one witness against the defendant is enough to require him to take a Torah oath in support of his defense before he can win.
Logic dictates that if the presence of one witness testifying against the defendant has the power to require the defendant to take an oath – even though he is in possession of the money claimed – certainly the presence of one witness testifying in his favor should have the power to exempt him from such an oath.