Refund on not taking possession of the flat

Booked a flat in Ghaziabad, paid 16.27L under 50:50 plan of the builder in 2014. This included a 3.09% service tax charged then. Agreement says 5% of the flat value penalty is chargeable by the builder if the buyer fails to take possession. The builder has sent the possession letter recently & is claiming 20% as the penalty... Does the builder have the right to do so, citing the downturn in the sector over these years? Second, can he charge GST now which was not in existence then (2014) How does the paid-up service tax get adjusted? Possession was scheduled to be given by Dec 2016...a penalty @ Rs.5/- per sq ft p.m. is leviable on the builder which in our case is for a period of 30+ months...Is this applicable if he is claiming a higher cancellation penalty (20%) on us? Fairness does demand that as today's situation at our end (inability to take possession) can be attributed to the delay in the offer of possession on the part of the builder. Informed, legal advise on the matter will be highly appreciated. Thanks & Regards