1) file complaint of cheating , criminal breach of trust against builder under section 406 , 420 of IPC
2) builder ought to have deposited UDS amount on sale of flat into BOI escrow account
3) if builder has failed to do so then BOI can sue the builder to recover the amount
4) you do not have clear and marketable title to property as no NOC was obtained from BOI and flats were subject to mortgage