Hello,
The reduced rate of GST is applicable at the option of the builder. If the builder has opted for the new reduced rates for ongoing projects, then he would be liable to collect GST only at reduced rates for any amount pending thereon. You need to check with your builder if he has opted for the new rates.
You won't be eligible to claim the input tax credit directly on your own. What the sales manager might have meant was that the ITC benefits available to the builder, because of change of tax from various taxes to GST, would be transferred through reduced prices to the buyers.
I hope that this answer satisfies your requirements.
Regards,
CA Hunny Badlani