Sa’if 18, Mechaber: Both the plaintiff and the defendant agree a loan was made. However, neither can remember how much was loaned. In this situation, the defendant does not have to take the Shevuat Heiset oath of denial. If the defendant knows he would not have borrowed less than minimum amount, although he might have borrowed more he must at least repay such minimum amount. He has no moral obligation to pay more. Others are of the opinion that in such a case it is not sufficient to pay this minimum amount. Rather, the defendant must reach a settlement with the plaintiff acceptable to both parties. In any event, the plaintiff may ask the court to warn the defendant that if he is lying, he will be subject to excommunication.
Ner Eyal: In these cases of uncertainty, a defendant is only required to take a Shevuat Heiset oath if the plaintiff is certain of the amount he lent but the defendant does not remember. Although under the black letter law of the halacha the defendant is allowed, in such a situation, to take a Shevuat Heiset oath that he does not remember how much he borrowed and have judgment entered in his favor, he does have a moral obligation to pay the amount he plaintiff claims. It is the certainty with which the plaintiff presses his claim that obliges the defendant to take an oath and that saddles him with a moral obligation to pay.
Where, however, even the plaintiff is not certain about how much he lent the defendant, there is no requirement for the defendant to take a Shevuat Heiset oath and he is under no moral obligation to pay anything. He cannot be blamed for being lackadaisical about his monetary affairs when the plaintiff was equally so. Those halachic authorities mentioned by the Mechaber who require the defendant to reach a mutually acceptable settlement with the plaintiff argue as follows: The defendant does remember that he borrowed something, even though he does not remember how much. He should therefore not be allowed to get away with it. Under the circumstances, he must try to appease the plaintiff with some reasonable payment.