Escalation Charges

I have booked a shop in Chembur in a New building whose construction was going on inthe year 2013. I payed full amount of the said property in cash as well as cheq as per norms. I also Paid the legal Stamp Duty and Registration for same After a year Builder demanded for VAT amount which I paid The said developer used to promise to give possession of that shop every year. But due to some reason the completion was delayed. This year in February 2016 the finaly construction was completed and we were waiting for possession . But Now the Developer is demanding for almost Rs 4 Lakhs which includes Maintainence, Development and ESCALATION charges . Out of which i am aware of Maintainence & Developement charges. which i m ready to pay But what are this Escalation charges which is almost 3.5 Lakhs ??? why shud i pay this escalation amount?? Is it compulsory??? wht if i dnt pay this escalation amount? will the developer create any problem in Handing over possession of said property ??? Kindly reply