Sa’if 17, Mechaber: The defendant is not required to take a Shevuat Heiset oath of denial in response to a claim about which the plaintiff is not certain. Accordingly, if the plaintiff says that he thinks he deposited a maneh with the defendant, or he thinks he lent him a maneh, or he thinks the defendant did not pay him back, but he is not sure, judgment will be entered for the defendant without him having to take a Shevuat Heiset oath of denial. There is not even any ethical requirement in this case for the defendant to exonerate himself ethically in the eyes of God and pay the amount claimed, because the plaintiff is uncertain of his claim whereas the defendant is certain of his defense.
Ner Eyal: Under Torah law, there is no requirement for a defendant to take an oath when denying a claim. It is upon the plaintiff to bring the required evidence to prove his case. If he fails to do so, judgment will be entered for the defendant without requiring him to take an oath. But seeing that plaintiffs are unlikely to start proceedings in court without at least some evidence in their favor, Rabbi Nachman instituted the rabbinical oath known as the Shevuat Heiset oath of denial, which every defendant must take in response to a claim, before judgment will be entered in his favor. This Shevuat Heiset oath of denial only applies, however, where the plaintiff is certain of his claim but not where he is uncertain.