Post Termination agreement..

Hello Sir, My name is deepak agrawal. I was working for company which is not registered in india. Might be they have proprietorship. So I have joined them at july 2014. After four years service i sent my resignation latter at march 2018 but they not accepted after that i have sent him again April 2018 and stop working for them. After leaving my job, one of previous client offer me job and i started working for him. Till now i made no agreement between each other. Now main point is come , that i demand for my experience latter and final salary settlement, they force me to sign NDA, and for getting my experience latter and final salary , i signed . In agreement words, that agreement issue to and signed at july 2014. After that company send me experience latter till feb 2018 not April. Now company sued against me that i stole their client according to NDA i can't work for him while i have hired by him already before signed me any agreement. So now i have following question.. 1. This agreement is valid or not because this is not make on any stamp paper or any company latter head this is on plain paper. Which is sent to me after my resignation latter . 2. Can i sued against him also for fraud to me that he enforce me to sign on NDA for getting experience latter or salary. 3. And post dated agreement is valid or not. 4. If i loss case then how much refund we need to give him . there is no mentioned regarding this in agreement. or we need to return only salary i get from his client. Please help me or suggest what action i can take against him. Thanks Deepak Agrawal