GST manipulation by builder

I had booked 3bhk flat at lodha upperthane casa ecopolis b wing flat number 601 which is underconstruction and expected possession date is september 2020 during the time of booking (october 2018) effective gst rate was 12% but with effect of new gst rules the gst for under construction property is 5% so i requested builder to reduce the gst but buider have refused it. now the builder is selling the other flats of my same building on lower gst of 5% by fooling customer and government authorities by giving 7% gst rebate and i am paying 12% as builder cannot have two gst rates for the same building which is under construction. builder can charge the new gst rate for new project or if builder is constructing new building in the current land parcel. so builder is cheating on the customer by charging high gst rate on older customer which is illegal and against the law I had already paid 90% payment towards flat kindly help I already filed complain in RERa. Now builder had issued defaulter notice and threatening me that if I don’t pay they will sell my flat