How to handle builder’s demands, cash request, legal pressure?

I am facing the following issues with a builder due to my lack of awareness of the rules: As per the agreement dated 14.12.2022, my flat area was stated as 1575 sq. ft., but it has now increased to 1680 sq. ft., and the builder has demanded an additional amount through a demand letter. He is also insisting on cash payment by offering a 50% discount. The flat sale is based on the super built-up area, whereas the carpet area is approximately 50% of the super area. The builder is forcing me to use his own advocate, who charges very high fees. Kindly guide me on how to handle this situation.