Builder is charging approx. 5 lakhs extra for possession of flat

We booked a 3 BHK flat in May 2017 with a mentioned super area of 1700 sq. ft (below is transcript from the contract): F. AND WHEREAS the Flat Allotee has applied to the Firm vide application date _____ with an initial advance of Rs ________ agreeing to the terms and conditions as set out in the application of the allotment of Flat No ___ on Floor ___ in Block ___under Category 3BHK having an approximate super area of 1700 Sq ft (referred to as the “ SAID FLAT”) and for allotment one of car parking. Now, while for possession he is saying that they have calculated the area wrongly on earlier occasion and now it is 1810 sq. ft so we have to pay the difference additionally. which comes to around 5 Lakh INR. He and his employee are so adamant that they are not even ready to talk it through properly and saying there is no option other than to pay this additional difference. My father met the builder few days back for this issue and he just dismissed him stating that nothing can be done; everyone must pay this amount and it is distributed proportionally to all tenants. Builder even has openly challenged my dad that if you go to CM (chief minister) also we can’t reduce the charges and will explain our difficulty to the authorities. After that also my father visited their office trying to meet the guy to talk him through, but they just ignored him and made excuses either he is busy or not in office and come back tomorrow - TOTAL AVOIDANCE Also, they are not able to give a concrete justification how the size has gone up – one of the occasions they said that it is for common lobby, lift and other common areas charges distributed among tenants, while on other occasion they said that while paying for paint job on the building they came to know about the actual size of building and hired an architect/engineer again to remeasure it. Furthermore, when we asked for a proper receipt/breakup of all the payments/load amount paid by us till date, including remaining amount to pay, they are reluctant to provide any record (official receipt) of the same and just telling verbally and asking us to note down in a paper and informing that once we pay then only they will provide the official details of charges after possession. Today they sent a SMS that we must take the possession immediately as the 2 towers (our flat is one of these) are ready for possession and maintenance fee is applicable for all "clients" from August 01, 2022. My parents are in huge dilemma and extremely tensed mental state now after feeling being cheated and looted and don't know how to proceed further. kindly advise whether we can take any legal action in this situation? we took advise from many know persons including RERA employees and everyone has advised to pay the money and take the possession as court case can be dragged/pending for years.