Sai’f 6. Mechaber: The defendant admits that he owes part of the loan. He then immediately pays the amount he admits. Such a defendant is exempt from the Modeh Bemiktzat oath of partial admission. He must, however, swear the Shevuat Heiset oath of denial that all defendants must swear before they can walk from a claim which they totally deny.
Ner Eyal: As a result of the immediate payment of the admitted amount in cash in court, the original claim is automatically reduced to the amount of the disputed balance. This reduced amount now constitutes 100% of the outstanding claim which the defendant entirely denies. A defendant who denies a claim in its entirety is exempt from the Modeh Bemiktzat oath of partial admission. This is because the Torah only imposed an oath on a defendant who disputes part of the claim but not on one who denies the entire claim.
The Talmud suggests that the reason for the imposition of the Modeh Bemiktzat oath of partial admission is to discourage a borrower, who knows full well he owes the balance, from delaying payment by initially denying he owes it, even though he intends to ultimately pay it when cash flow permits. The irrevocable consequences of taking a false Modeh Bemiktztat oath of partial admission would deter him from employing such delay tactics. But the Torah does not suspect a defendant of having the gall to deny the whole loan, unless he really does not owe it.
The rabbis of the post-Mishnaic era, however, were no longer convinced that a defendant would not have the gall to deny the whole loan merely to buy time. In addition, there is a presumption that a plaintiff would not institute legal proceedings unless, on the face of it, he had a good faith case. Accordingly, the rabbis imposed on a defendant who denies the entire claim a less severe form of oath, known as the Shevuat Heiset oath of denial. The purpose of the Shevuat Heiset oath of denial is both to dissuade the defendant from denying the claim merely to buy time and in order to rebut the good faith claim presumption.