Mahaveer group Reddy structures pvt ltd Bangalore - GST CONCESSION REFUND FOR PMAY BENEFICIARY

I purchased a *bhk flat (SBA *** sq.ft), in mahaveer group (turquoise), Bangalore (m/s. Reddy structures) in jan **** & also entered into sale agreement, at the time of booking & agreement, builder said, I will get gst concession of *%, if I'm eligible for pmay subsidy. Subsequently, i got subsidy in April **** ,& also submitted bank letter to builder, in this regard. After so many follow ups, builder side was saying, gst concession of *% will be adjusted in final settlement at time of handover, Now recently, when I contacted them, they say, we opted for old gst rates @ *% with ITC, as one time option in May ****, hence no concession available to you,!!! I booked flat in Dec **** & entered into agreement in jan ****, but builder has to exercise option on May **th ****, if so, builder should have informed the same to me at time booking. But they gave false information to me & now denying the concession is not fair business!!!!. The issue is, I'm paying extra *% gst to govt, but builder is getting benefits of that *% in form of ITC for the past *.* yrs, Moreover, now date of completion is extended upto ** sep ****, almost * years delay in handover, everywhere normalcy returned, but works are progressing very slow, which already caused huge financial & time loss to me!!! Look into this issue empathetically & Kindly do the needful asap!!!! Looking forward for fair & ethical reponse!!!!