Pre-possession demand letter

I purchased a flat through a construction link plan. As per the plan, I will have to pay last 3 percent of total amount on the possession of the property. The builder has issued a pre-possession letter and demanded final payment as per the construction link plan. By visiting the site, I don't find the progress satisfactory as there is still a lot of work to be done. The builder has already delayed in offering the possession. It has been 9 months delayed and as per the agreement the builder is supposed to pay the penalty (Rs. 5/Square foot). However, while asking to adjust the penalty in their final demand, the builder says it has become null and void since there was a small amount of interest due on my part. Also, the builder is charging extra Electric Charges apart from what is mentioned in the agreement. One is Electric Connection charges and the other one is "Electric Connection Meter Charges". I feel cheated. Please suggest what should I do. If I don't accept the offer of possession, they will charge interest. Also, they have mentioned 90 days to hand over the flat after I pay the all my dues. I think they should not take more than 45 days to hand over the flat after receiving the final payment. I think it is just a trick to avoid a penalty for further delaying the possession