Saif 13, Mechaber: The rule that one who is obliged to swear but is unable to do so must pay only applies when the defendant is obliged to take a Torah oath. It does not apply with respect to a rabbinical oath.
Ner Eyal: The following cases are examples of situations in which the inability to take an oath would not result in a judgment against the defendant because the oath is imposed by the rabbis and not by the Torah:
Disputes concerning land, servants, and promissory notes; disputes in which the Modeh Bemikzat oath of partial admission imposed by the Torah does not apply, such as when the subject matter of the dispute is different from the subject matter of the defense. An example of this is where the plaintiff sues for the return of a bushel of wheat and the defendant responds that he returned him half a bushel of barley.
Another example of a case in which the Modeh Bemikzat oath of partial admission imposed by the Torah does not apply is where the amount admitted is less than a minimum amount of two kesef silver coins. Yet another case in which the Modeh Bemikzat oath of partial admission imposed by the Torah does not apply is the case of “heilach,” when the partial amount admitted is immediately paid by the defendant.
This has the effect of reducing the claim from the original amount claimed to the amount of the remaining balance in dispute that now constitutes the entire claim. The defendant disputes this remaining balance in its entirety.
The rule is that a defendant who denies the claim in its entirety is exempt from the Modeh Bemiktzat oath of partial admission. Yet another case in which the Modeh Bemikzat oath of partial admission imposed by the Torah does not apply is where the amount claimed is not something that can be counted or weighed.
Although the defendants in all the above situations are exempt from the Modeh Bemiktzat oath of partial admission imposed by the Torah, the rabbis imposed on them a Shevuat Heiset oath of denial before judgment can be entered in their favor. Because these oaths are of rabbinical origin, the rule that one who is obliged to swear but is unable to do so must pay, does not apply.
Another example of an oath imposed by the rabbis, this time on the plaintiff rather than the defendant, is the case of a worker who has not been paid his daily wages. For reasons of public policy, the rabbis allow a day or night laborer, whose services have been hired by a contractor and who worked in the presence of witnesses, to swear that he has not been paid and collect his wages. In this cases too, even though the rabbis require the plaintiff to take an oath before he can prevail in the litigation, they did not apply the rule that he who cannot swear must lose.