Sanhedrin 24 - Retraction of Judge Selection
Relatives and those guilty of certain misdeeds are ineligible to serve as judges. However, if both litigants agree to accept their verdict, then even they can preside at a trial.
If a litigant declared to his opponent, "My father is acceptable to me to act as a judge," or "Your father is acceptable to me to act as a judge," or "Three cattle herders are acceptable to me to act as judges," but afterwards changes his mind and wants to retract, then Rabbi Meir says that indeed he may retract, but the Sages say that he may not.
If he wants to retract before the verdict is reached, all agree that he may. The argument is only after the verdict is reached, and the law in this case follows the Sages - that he may not retract.
Art: Pietro Ligari - Portrait of the Artists Father