Builder charged interest

Hi, I have buyed one flat in Pune last year. When I buyed, it was under construction and builder suppose to deliver the project in June'17. Initially due to some personal work I delayed first payment by one month. Builder changed an interest amount on that because of delayed payment. The interest amount is around 1 Lac. In each of the demand letter builder keep on mentioning the same amount even after my repeated request to remove that interest amount. In fact I met with the SPOC and clarified, he told he would get back to me. but never replied. I asked same over an email too. he never replied and said that you pay as per you demand letter and for interest he will take care. Now the builder has delayed the possession by 4 to 5 month. When I paid all the amount as per agreement (till possession)..he is behaving cunningly and asking for the interest amount. Today again I visited his office and he told me that I will have to pay .. but again he will discuss with his superior and get back to me. This is really irritating me and delaying my possession. I want to know 1. Can I take a legal action against him as builder failed to delivered the house at per agreement & forcing me to pay the interest? 2. Can I complain against him in RERA for this harassment? if yes what is the procedure? 3. Can I ask for the an entire amount as he failed to delivered the flat on time? I am not in a position to pay any amount extra.. If they force I have to withdraw .. Kindly advice !!