GST on the flat booked before GST implementation

I have booked flat in Oct 2016 and 80% of payment has been made before July 2017 with 4.5% service tax and 1%VAT. Builder is asking me to pay GST @12% on the remaining base payment which causes me extra burden of approx 60,000 INR on the total cost of the flat. I have got an article from CBEC site(http://www.cbec.gov.in/) for more clarification regarding this in which it is said as builder is going to get more benefits on the material purchased after July 2017 as compared to previous regime of tax so they should pass on benefits to the customers and should not ask customers in situations like me for extra GST. If builder does ask for extra amount then it is against GST rule 171. Mentioned here: http://www.cbec.gov.in/resources//htdocs-cbec/gst/reduced-liability-tax-home-gst.pdf I have discussed this with builder but he is saying until possession he will not be able to passon the benefits to me becoz as per him it's unclear to him right now how much benefits he should pass on. Can someone please help me out.