Builder cannot use any FSI in such cases
That FSI belongs to the society even though conveyance is not done
You have to write to the BMC to not sanction any plans of the builder which seek to use such FSI
If despite that any such plans are being sanctioned then you can file a Writ Petition seeking mandamus against BMC
Ideally the society has to file a civil suit seeking a declaration that it is owner of the FSI and its future increases