I am the inventor of a mechanism that was assigned to a client under contract. My client breached the contract for non payment at the end of the contract. I had tried negotiating for two years with both attorneys as well as negotiators. I had sent collectors after the client as well . At the end of some of my demand letters I warned the client I would revoke my assignment. This was my only recourse. I was able to revoke both power of attorney as well as the assignment rights. the assignment was revoked on the basis of wrong title of assignment associated with the patent as well as breach of contract. Ater 6 years of no action the former client had his lawyer send me a demand letter to regain assignment rights. they are threatening damages. do they have a case?