Carpet Area less than that in the Registration Agreement

Sir/Ma'am Last October 2014 we had booked a flat which was nearing completion. We were promised that we will receive it by Dec 2014. When December went we were promised March 2015. When march too passed we went and asked the Sales head to give us in writing and so on their letter head they said August 2015. So at the start of August we went to take measurements for furniture as we hoped that they will give us by end of August 2015. But while measuring we realized that the area is not as mentioned in their brochure and so we decided to measure the entire flat and we came to know that the flat is 43 Sq ft lesser than the Carpet Area mentioned to us. We brought our concern to the Sales via mail and personally meeting but they were not ready to accept. As per them the difference is because of plaster thickness. And this variation is normal. Since they refused to listen we filed a complaint against them in a government department. And they too found the flat area less. They sealed our flat and gave builder a notice. But now the builder is threatening us that he will not give us possession and neither compensate. Sir we have taken a heavy loan on this house. And along with EMI also paying rent past 1year due to their false promise. We have already paid 95% of the flat amount in addition to registration and stamp-duty. The only amount remaining is 5% which as per agreement is to be dispatched by the bank at the time of possession. Sir, is it wrong to ask the builder for giving us lesser carpet area than promised? Even after paying 95% amount, paying the government for registration the builder says this is his flat and he will not give us possession. Please please please suggest what do we do. Thank you.