Cost of Apartment increased by builder even after booking at lower rate ?

Hello,

My father booked a flat at native place @ 1800 PSF 1 year ago. At that time he had not done the registration, but signed agreement with builder (Agreement says that we have done full payment and only additional payment will be Stamp Duty, Registration Fees, MSEB and Maintenance). As per today’s rate, apartment cost is 2400 PSF. Now, when we want to do the registration, the builder is asking us to pay as per current rate.

Can we take any action on the builder?

Thanks in advance…