Do I get a signed termination agreement (if possible)? Can I withhold their funds until this is resolved? Or can I just return the funds to the bank and provide a notice of termination letter? Will I be able to pay a construction lawyer review the notice before I send it or will it be more complicated than that?
The answers to your questions will depend heavily, if not exclusively, on the terms of the contract. You should ask an experienced construction law attorney to review your contract and provide you with advice.