This is a difficult matter for me to offer any advice or response. First off, I don't really know the details of the situation, nor am I fully familiar with all the facts. Anything I say would be uninformed and liable to error. JVO is really not the appropriate forum for such an inquiry.
You should know that Jewish law is far more stringent on proofs than common law: it generally requires that there be two disinterested witnesses who see the action as it is being done for any penalty to be imposed - one is not enough. It also requires that the witnesses know the intent of the person, warn that person before the action is taken, and that the person understand the warning and the consequences, and still proceed in full view of the witnesses. I can't imagine that this standard would be met from the little you tell me; Jewish law and tradition would require you in that case to be silent and take no action.
Looking at what you say in regard to the prior behavior, you are telling me that you have heard something now - AFTER you hired the rabbi, that occurred before you hired him. I would think that it is too late to consider that now: either it should have been discovered and known in the interview process (and if you hired the rabbi knowing it, you can't raise it now), or it is water over the bridge.
More: this is not fact, but what you are 'hearing'. Apparently you can't personally swear to it, it was not established by a court (Jewish or other), it is not public knowledge, and you were not a witness to it, so it is not an established fact, and to act on it in any way or speak of it is inappropriate.
You tell me that you (and perhaps some others on your committee) feel that a contract was entered into by the rabbi without authority; again, without much more information it is impossible to determine what really happened. How did the contract come into existence? What was the circumstance? Could the rabbi have been given signals that this was appropriate and he should proceed? If no, was the rabbi ever specifically told that his role did not provide for him to enter into contracts? Did anyone at any time ever tell him that he would be subject to penalties if he did? Did anyone warn him and then see that he entered into the contract despite the warning and knowledge that he was not permitted to do so? There are too many questions to be able to give you a meaningful answer.
I think that this situation is not so clear cut as you may have thought. It is quite possible that the rabbi showed poor judgement, or even overstepped his bounds, but Jewish law does not impose penalties for that type of behavior.
Apparently you, or someone, has brought this to the rabbi, and he has not agreed that he was in error. It is possible that he is correct, and that what you think you know is not the whole story, just as much as it is possible that your view is correct and he is in the wrong.
If your goal is to mend fences and continue to work with this rabbi, then what is required would seem to be a closer working relationship, with explanation, discussion, and an open give and take leading to agreement on what the rabbi's role and authority are in the congregation: it is not a group of people sitting as judges over the rabbi and imposing penalties for what they don't like. The relationship is, after all, supposed to be a Brit Kadosh - a holy covenant, and not an employment contract for hourly wages and tasks by the worker dictated by the employer.
If the goal is to punish the rabbi, and/or eventually to assure that he leaves, this will become an ugly, divisive, and destructive situation in the congregation, and will resound to the congregation's reputation for many years to come.
If you wish to pursue this in a formal manner, it is likely that you need to do so by going through the CCAR and the mechanisms established by the URJ. If that is unsatisfactory, you can ignore Jewish law and take the matter to the secular courts, but know in advance that because this is a messy process, and one that airs all the dirty laundry, neither party is likely to be happy with the outcome, nor to feel fully satisfied with it, and it is almost certain to create ill will and hard feelings among congregants.
Again, there is no answer possible because there is not enough information to formulate one. I hope that you and the committee members can deal with this in the most positive and mentschlich fashion and bring closure and peace to all.
Rabbi Joe Blair