Gst for apartment booked before 1-july-2017

Hi,

We booked apartment back in Sept-2014 and have paid around 80% amount before GST kicked in.

Now builder is asking us to pay GST on the balance amount.

Reading some articles on-line,it seems builder can not charge GST for apartment that is booked before 1-July-2017

(ref: https://www.ndtv.com/business/booked-a-flat-before-july-1-builders-cant-charge-more-for-gst-1732327)

Can any one shed some light if that is true ?

Thanks,

Milind