Gst on property

Hi sir 1. I booked a flat in jaipur by giving 15% of the flat payment to the builder . The flat is of 483 sq feet . I booked it in dec 2016 . Builder has not started the project till yet n asking me to take another flat in his another project . He is not ready to return my payment. 2. If I take another flat in his already build up project . He is asking me for 12% Gst n the flat size is 859 sq feet . When I told him that the building is already constructed and you should not charge me gst. He told me that he has not received occupancy certificate so he will charge 12% gst. My question is- his project is 90% sold out. Building is complete . Should I still have to give gst 12% or 8 % as the flat size is less than 60 sq metres . Second question - As builder took 1.15 lac in cash n 75000 in cheque while pre booking the initial flat (where he has not started the project ). He told me that he won't return the cash payment . He can only return the cheque payment . In this condition , what should I do