From Official Wiki of the Renaissance Kingdoms

A fine in pounds can be imposed on your character at the verdict stage of their trial in court. The fine must comply with the Rules for the Administration of Justice (out of character limitations on sentences), and if it does not comply then the player of the sentenced character can request the annulment of the sentence.

According to article 5 of the Rules for the Administration of Justice, if you are unable to pay the fine imposed by the court, you can appeal to the administrators to clear the negative, by posting in the relevant county topic in the Justice Records sub-forum of the International Justice forum. If you try to trick the administrators, they may punish you.