Sell under-construction flat process

Hi, I booked a flat in Aug 2014 with Runwal in Mumbai through Proptiger the registration of which happened in May 2015. I am looking to sell this flat as servicing this loan is no longer a priority (want to reduce my debt burden), what with property prices staying stagnant (the purchase was more a panic reaction than genuine need or investment due to property market bubble when I booked it). I find myself handicapped reading legal documents so I have not looked at the registration documents and other terms and conditions between myself and the builder and me and the bank. I sent a mail to builder if I can sell my under-construction flat (possession due in Dec 2018, paid around 85%, mostly thru loan) and the response received stated I can only sell it post possession, implying there is some lock in period. They also refused to buy back which is understandable. My question is, is this lockin period legal? If I sell it, would it be short term capital gain or long term. How do I go about selling it. Thank you for your time and guidance.