Amazon is not giving cashback of 1500. How to proceed ?

I've not received a cashback of 1500 from Amazon. They mailed me saying that they transferred it to my bank account on 5th Jan 2017. Bank is denying and asking me to get transaction number from Amazon. Amazon is not giving transaction number saying it is confidential. Amazon has wasted 4 months of my time and effort in following up. I've provided them my bank statement as proof that I haven't received the cashback. But they are lying everytime and not doing anything. I have all the proofs in my Gmail. I want to file a case against Amazon. What relief/compensation can I expect in this case? What would be the procedure and time period required for it? What would be the lawyer's and procedure's​ fee?