Ownership of common garden after BU permission ?

While purchasing of bunglows at very well known area and builder, builder has shown common garden , road etc. Builder had taken NA permission of this bunglow scheme .In NA permission common facilities right given to members/ buyer of bunglows. But during procedure builder had carried out sub plot land and common road sale deed with member, but builder had not mentioned garden area in sell deed. But in sell deed builder has kept his right on garden area. When builder got BU permission from authorities, builder/ authorities had mentioned NA permission letter reference. Builder also mentioned NA permission letter reference number during possession to bunglows buyer. Now Builder claiming ownership of common garden and want to sell / build flats. How can builder overrule NA permission or government authorities? Builder telling that if we bunglows owner want these common garden on our name we have to pay stamp duty (where we have already given lupmsum amount to builder while booking bunglows for legal / stamp duty etc. charges)