Can we still sue if we signed a contract agreeing not to?

Question Details: My husband worked for someone in CA just over a year ago. They signed a contract to pay him hourly and to give him a retainer of $2500 prior to him flying out there. They didn't actually pay him the retainer until he was physically there and after about a week of him working. After he completed the job and came back home, he invoiced the client monthly but they never responded until a few months later when he started the process for small claims court to retrieve the balance owed. He sent them notifications that if they did not pay by a specified time, he would sue. The day before he was going to submit the application, they said they would pay him if he signed a "do not sue contract". He signed it and they promised that they would pay by the next week. However, they still have not paid despite being given monthly invoices. Can we take them to court for failing to uphold the second contract and subsequently, the initial contract?

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