Due to additional burden of GST, can i cancel my flat booking

I had booked a flat on 29th April 2017 by paying 1.00 lacs booking amount. On 9th June, I received mail from builder regarding implementation of GST from 01 July 2017. On 30th June late evening, I received an email communique giving summary of key aspects relevant to GST i.e Sale of Flat under construction (i.e. sale before issuance of completion certificate or before first occupancy, whichever is earlier) will be liable at 12% GST (provided the value of land is included in the amount charged). Therefore, the new applicable taxes will be collected on that portion of your flat for which collection is received and/or invoice is raised and milestones achieved after the appointed date. “Considering the invent and additional burden of GST, other increased charges and also some of my emergent personal issues, I requested to cancel the booking of my flat and refund the booking amount of Rs. 1.00 lacs” Builder has written that entire amount shall be forfeited whereas in the price sheet it is written as "Cancellation charges - amount upto 20% will be forfeited" Can I cancel the flat booking due to invent and additional burden of GST? If yes, how much refund am I entitled to get?