Gst

I am purchasing a flat in Ranchi, Jharkhand say, for Rs. 50 lakhs. The seller, Mr. A had purchased the underconstructed flat under the service tax regime (was liable to pay service tax) and currently has payment due to the builder of Rs. 20 laks out of the total consideration of Rs. 45 lakhs. The seller, Mr. A had made payment of Rs 25 lakhs along with applicable service tax to the builder, Mr. B. Now I, the buyer, am entering into a tripartite agreement for purchase of the flat, whose construction is completed as of now, with the builder, Mr. B and the seller of the flat, Mr. A, say for Rs. 55 lakhs. I am paying the balance amount of Rs. 20 lakhs plus GST @12%, on behalf of the seller, Mr. A, to the builder, Mr. B and the remaining amount of Rs. 35 lakhs to the seller, Mr. A. My questions are- 1. Is GST applicable @12% on the payment of Rs 20 lakhps to the the builder, Mr. A? 2. Should the GST amount paid by me to the builder, Mr. A, be shown in the Tripartite agreement and also the Sale deed? 3. What if the buider is not ready to show the amount of GST ( which I am paying to the builder, Mr. B, through cheque in his name) in the tripartite agreement as well as the sale deed? Is he collecting extra money in the name of GST?