Cancellation Process and refund

Respected Lawyers, I have bought under construction flat in oct 2015 and made initial 20 % payment and was remaining 80% from HDFC home loan as I have already approved sanction letter from HDFC bank which was going to expire in Jan 2016. At that time project was not approved from HDFC bank but builder told me that it is already in process and will be approved in Nov end of 2015 so I have paid initial 20% payment and got the allotment letter. Till today May 2016 the project is not got approved from HDFC due to some NOC and builder is forcing me to take loan from other banks. I find the behavior of builder doubtful and told them to cancel the flat and refund the 20% amount on 10th May 2016 through email as asked. He is now asking me to wait to refund the amount till they got new allotment in their project. My Question: 1. How much time I should wait for refund? 2. I have wait for 8 months to get it approved from HDFC and now they are asking me to wait for new allotment in their projects. For delay in payment they charge for 18% interest Can I force them to pay 20% amount with interest of 18% as I have to wait for 8 months? I am first time buyer please advice.