Could you give a few basics rules according to Jewish Law regarding monetary disputes.

All questions involving monetary disputes cannot be addressed properly unless both sides are heard (in this sense, these questions are different from questions of kashrut, for example). The general principle in Jewish Law is that receiving a benefit requires the person who has benefitted to compensate the person who has given him the benefit. There is no assumption of "volunteering" unless specifically stated or in a situation where volunteering is the rule rather than the exception (someone who invites friends over for kiddush can't give them a bill afterwards). If the service received was not requested, the person receiving that service can return it and not pay for it- or, if he can demonstrate that he could have received the same service for less money- his payment is no more than the value of the benefit he received. If the service was requested then compensation is the market value of of the service. To deal with a specific case, you can always say you want a Din Torah. Find a Bet Din L'mammonot in your area of the country - they exist in many cities in Israel.