Relieving letter not given by Employer though I seved my notice

I have resigned from my previous employer(not MNC) recently where I had worked on contract(C2H) basis. I'm deputed to the client from my previous employer and I worked at the client location. Since it is a contract position, I looked for another good opportunity after 1 year. My employer has the notice period of only 15 days and when I submitted my resignation, they said me that it is purely client decision. So when I approached the client for the same, they asked me to serve for 30 days and they approved my last working day. Now the thing is that my payroll employer didn't commented on resignation mail which got approval from the client. when I contacted(on phone) them to comment on the mail,they said that it is not needed since you have client's approval. Now I'm out from that Employer and joined in other company. I didn't got my last 2 months salary and the relieving & experience letters. when I asked them, they are saying unrelated reasons like "you have attempted to break the company policies and so the complaint team asked us to hold the salary & experience letter". But I didn't disobeyed their policies. Please suggest the better approach to get my salary and documents. My present company haven't asked the relieving letter and if they ask it in future, can I show the accepted mail from my client? Is there a chance for my previous employer to proceed legally on my present employer on how they had taken the candidate without relieving from their side?