Developer asking for more payment on account of increase in area

I have purchased a flat in Noida Sector 76, Uttar Pradesh in May 2013 in resale. In 2013 itself, the construction of flats were ready & internal finishing work was in progress. The possession was supposed to be in 2014, but if you are aware because of NGT issue case in NCR, most flat possessions got delayed. And since I purchased the flat in resale, builder revised the possession date in Allotment letter, so late penalty is out of question now. Since all flats were ready & NGT case results were still pending in beginning of 2015, builder offered a conditional possession without completion certificate from Govt. authority. Builder played a safe game by asking all buyers to sign an agreement in favor of builder. I made all final payments, had bank loan so got NOC from bank as well and finally got possession in July 2015. I started living in the flat including many such families basis offer of possession without registry/deed. After 3 months (in Oct 2015), NGT story got closed & we heard good news that now house registry are in process. Now in Dec 2015, after 5 months of possession, suddenly the builder had issued a letter (with a shock to me) claiming that there is an increase in build-up area by 59 sq.ft. and we need to make a payment for the same at original purchased rate. Although they didn't mention the amount or time of payment, but when I calculate it comes to be approx 2lacs. My flat is of 1220sq.ft area and I already paid 45lacs to builder & recently spent 3lacs for registry stamp papers. I understand there is a clause in their Allotment letter (every builder had this clause) that we need to pay for any extra area we get. But I failed to understand, since from time of my purchase (i.e. in May 2013) & till June 2015 (at time of possession & final payment) why builder didn't mention that there is an extra area of 59 sq.ft. and it will be chargeable. I have 2 questions from legal experts, ever there is a clause in their allotment letter, can builder claim this payment after possession & just before the time of registry. Can I file a complaint or take any legal action against this act of fraud. Regards, Sachin