Delay possession because of delay in oc and sudden increase in size on intimation of posession

Kind attention: Learned / Knowledgable Law Friends, Subject: Delay of more than 4 years in Possession of commercial unit and then sudden increase in size of the Unit at the intimation of possession letter. We had booked an commercial retail unit with a Builder / Promoter in NCR, in 2008 and nearly settled most of the payment by 2011, however, the Promoter / Builder could not get the occupation certificate in 2011 because of their own faults and in spite of our regular requests asking them of the precise possession details they never informed us of the factual information or even any increase in size of our unit in the building which was ready in 2011. They informed us of the receipt of OC in the year 2015 and therefore we were writing them continuously to inform us why the size has been increased in year 2014/15 as the building was ready in the year 2011. We took the possession of the Unit in Nov.2019 and after that the COVID prevailed. The unit is still lying empty and we were not compensated of any amount for the delay in possession and even charged for the changed / increased size of the Unit. Now, we are being charged the CAM since the possession was taken even the unit is lying vacant. Please advise what can be done as the Promoter / Builder had a good ride on us by playing with our money. Thank you and best regards. Rakesh