GST Charged by Supplier of TDR in Tax Invoice - Goods and Services Tax - GST

Respected Seniors, Facts of the Case : My Client is a Builder/Developer. He has purchased TDR from LLP. This purchase of TDR transaction is Exempt for Transferor as per Notification No. 4/2019, dated : 29-3-2019 . On this Purchased TDR, Builder/Developer is liable to Pay RCM on unsold Residential units at the time of Occupancy Certificate. While issuing Invoice for TDR, Transferor (LLP) has issued Tax Invoice and levied GST @ 18% and uploaded the same invoice on Portal in GSTR-1. Contention of TDR transferor is that he has generated TDR from Surrendering Land for Road Construction. Hence he is liable to Collect GST @ 18%. Querry : 1. Is there any specific bifurcation in GST Law where GST on Transfer of TDR is liable @18% forward Charge ? 2. Will there be any liability under RCM on unsold units at the time of OC, since GST @18% is already collected by landowner and paid to Government ? Thanking you in Advance