Sector 75, Noida

I have purchased a flat in sector 75 worth 47.8 lacs (2+1 bhk) under 50: 50 plan on 22nd April 2017. As per BBA, I was supposed to pay 10% at the time of booking and 40% within 30 days of booking and remaining at the time of possession. Now as per plan, I have paid 15% at the time of booking (as builder asked me to do so). Further, I waited for the demand letter, but after several reminder over phone as well as message, builder delayed the demand letter and finally issued dated Jun28th, and then bank sanction the 40% (including mine portion) to the builder before the due date as per demand letter. I have obliged with the payment. Now after 6 months when I asked for the final demand letter he has falsely charged 98000 amount as late payment and included this into final demand letter. Also, he has charged 12% flat GST on remaining amount without giving any ITC benefits. can you please suggest better course of action as I can see this is not worthy to charge late payment as well as GST without any ITC. Regards, Ankit