Builder Resorting to fraud, delaying project and not returning mo

I bought a flat from a builder in Delhi in December. The flat was under construction back then and it was promised that the flat would be transferred by 10 August 2015 (The date has already passed). The builder and he also signed a sale agreement for the same mentioning every detail (Like the money already paid, occupation date of flat, details of cheques paid by me etc). I had already paid Rs. 29 Lakhs to the builder and I have all proofs of the same like my bank statements and also the cheques are mentioned properly in the sale agreement. Now we have realised that the builder is deliberately not completing the construction and on being asked to return the money paid by me, he is giving redundant excuses. I have also come to know that the property is a disputed property and he still signed a sale agreement for it. He owes me Rs. 29 Lakhs and he is insisting on paying the money in parts (cheques of 5-10 Lakh each over 2-3 weeks). He is asking me to cancel the agreement after payment of first cheque, a proposal that I have outrightly denied. The builder even threatened me by saying "do whatever you can". I am facing mental trauma and visiting his office almost daily to get my money back. Another option that he is proposing is that he choose another flat already constructed by him in some other area and he will transfer the flat immediately. I do not want any other flat. All I want is my whole money that I have paid him to be returned back along with any interests that may have accrued. What legal action can I take? Are there any precautions as well which are to be taken? Where can I complaint for the same? I would be highly obliged if I can get any advice.