If you cannot work this out by negotiating then I suggest that you file a small claims action in Superior Court. You do not need a lawyer and cannot have a lawyer represent you in small claims court. It is always a good idea to consult with a lawyer. A lawyer can help you think through the issues and procedures.
Consider buying the Nolo Press book available at nolo.com that deals with small claims actions in CA. You can order over the Internet for hard copy delivery or you can download it instantly.
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Many times a letter from an attorney can be very pursuasive in convincing people to do the right thing. This would be the type of thing that we handle in a consultation, in which you get to meet with me for an hour, and the fee for this type of consutation is $200.00.
The other alternative is to file a small claims case, where you can your case to court quickly and economically. There is a self-help center that is part of the small claims process that will be of assistance in working your way through that system.