Everything in a contract is open to negotiation, even your legal right to sue if the contract goes awry. For example, in the case of American National Bank v. A.G. Sommerville Inc. (191 Cal. 364, 216 P. 376 (Cal. 1923)), Sommerville sold Tomlinson two cars. The contract stipulated that Tomlinson could not “attack the validity of the contract” if Sommerville sold the contract to a 3rd party. Sommerville sold the contract to American National, who tried to collect payment for the cars. Tomlinson refused to pay, claiming he never received the cars. American National sued. The Court found for American National. They found Tomlinson’s testimony about not receiving the cars inadmissible because he couldn’t attack the validity of the contract!