Responsa for Kiddushin 99:12

Q. A and B placed their dispute in the hands of two mediators subject to a final decision between the two, of C, the third mediator. They agreed before an authoritative court, that should one party refuse to abide by the decision of the mediators, that party was to pay five marks to the cemetery fund or to the synagogue. They deposited valuable pledges with C to enforce their agreement. The mediators reached a decision on the second day. A, however, refused to abide thereby claiming that, at the time of the making of the agreement, it was understood by both parties that the decision of the mediators was not to be postponed till the second day. A brought two witnesses, one of whom testified that such was the understanding; the other witness was not sure whether the condition was made at the time of the agreement or subsequent thereto. C and the subscribing two witnesses denied A's allegation that such a condition was made at the time of the agreement. The Rabbis, therefore, decided that A must pay the five marks fine. A, then, changed his mind and expressed his desire to be bound by the decision of the mediators. Must he still pay the fine?
A. As soon as A refused to accept the decision of the mediators, he became obliged to pay five marks to the sacred cause. Therefore, A must pay the five marks fine.
SOURCES: Pr. 975; Cr. 289; Agudah Sanh. 5.