[In the times of Chazal and beyond, there were many situations in which a person would make an oath of Torah or of Rabbinic origin. There was also a concept of accepting a cherem, whereby the one who accepts the cherem does not swear but accepts a curse of sorts upon himself if that which he asserts is false. Nowadays, batei din to not administer oaths and to the best of my knowledge, charamot are not generally used. As we see here, 140 years ago in Eastern Europe, at least one beit din was dealing with an interesting question related to the use of charamot.]

[Reuven claimed that Shimon owed Reuven’s father 800 rubles, as Reuven’s father announced in front of two witnesses, and now that Reuven’s father has died, Reuven is to receive the money. Shimon said that he owes only 300 rubles, and that he had informed Reuven of this fact during his father’s lifetime. In fact, said Shimon, if Reuven will accept a cherem denying that Shimon had told Reuven as above, he (Shimon) would pay the entire 800 rubles. They signed a document confirming the challenge and strengthened it with a kinyan sudar. The question is whether this arrangement is binding, as Shimon now wants to get out of it. The local rav/dayan posited that since a cherem is basically a form of oath, one cannot create a binding cherem on a claim that does not have intrinsic significance. Specifically, even if Reuven were to admit that Shimon had denied the oath during his father’s life, that would not have exempted Shimon from paying (there was no claim that Reuven or his father had admitted that Shimon was correct). Therefore, giving credence to such a cherem is "making a condition that contradicts the Torah" law of not making valueless oaths. The local rav asked the author of Beit Yitzchak for his opinion on the matter.]

The fact that the cherem is on a point that does not have a natural impact on the case is not relevant. A sh’vuat shav (a frivolous oath), which is forbidden, exists in cases where the oath is trivial because the subject is obvious, not because it does not include a legal ramification (see Rambam, Sh’vuot 1). It is true that dayanim would never have instituted the cherem as Shimon did, but if Shimon proposed it, it is not forbidden for him to accept the cherem. Therefore, a monetary condition that is dependent on it need not be invalid.
The question is whether the conditional self-obligation Shimon accepted is an asmachta (an exceptional conditional obligation that one accepts because he does not expect the situation will transpire). One thing that makes the obligation binding is that Shimon handed over the money to a third party to be given if and when the cherem is accepted. Furthermore, this is not a full asmachta, a concept that the Ramban promotes regarding one who accepts to pay based on the testimony of a relative of one of the litigants. The Ramban says that while this acceptance contains elements of asmachta, which make a kinyan necessary, this limitation can be overcome by a regular kinyan sudar because it is not a full asmachta. The same is true in this case [where it is not a full asmachta because they just alter the rules by whose means a dispute will be decided].