Need help in filing case against Interior designer

I have recently done with my interior based on a reference. But the interior person took more money by showing false numbers and finally when we calculated the amount based on the woodwork done and according the rate we set in the agreement, he took a lot of money extra. Now when I am asking him to give my money back, he is showing all false numbers in computation of the wood work area. I even got it checked with a civil Engineer who works for Central government and even he mentioned that his calculations are all wrong. Lucky we have the aggrement signed on his interiors letter head. So following are the concerns: 1. He extorted extra money with false numbers. 2. There are lot of damages on the wood work and the painting is not proper. It was a disappointed with the quality as well as the house was our dream. 3. The material he use and the one mentioned in the agreement is different. He also added few additional items in the cost to cover the money he extorted even though its not in the agreement. Also his interior is not registered even though he advertises it in web sites, advertisement boards, visiting cards, etc. He claimed to be working for 10-15 years without registering, so he hasn't paid any tax. So all the money he extorted from me are tax free and thus black with him. Can you please guide me what all can be done to get my money back and also the justice with the quality of the wood work with some compensation. How long does the case run if at all? Also can I file a direct case with the police? Also how can I file a complain for income tax as he is running a company earning lakhs and even crores for years without paying tax. Thank You, Bapineedu