Hi, I am the prime contractor on a project. The client and I decided to mutually terminate the agreement. We found their demands to be beyond the scope of work and they would like to bring in another contractor. We have correspondence issuing the statement that the contract was not terminated due to nonperformance or compliance with the contract. We would like to issue a release so they do not attempt to claim failure to perform and back charge us for the other contractor work. What would be the legal document they would need to sign?