Cancellation of development agreement

My father in urgent need of money , dealt with a builder about land where he signed on development agreement about 10 dismile out of total 14 discmile . He asked for 50lakh rupees as he had to pay old debt . He received money on the point that if he wishes to return money and cancel the agreement he will be charged 6%p.a. interest rate , otherwise the sum of money would be adjusted in his 43% share of built area if the agreement continues . At the first hand , all the above was on a stamp paper document ; but later was signed as development agreement . It has been a year since agreement . Now , we came to know that ..the builder with his lawyer , playing with words has fooled my father with agreement of total land area along with improper rate of land @sq.ft. for flats constructed ; which means my father will not be left with any flat after completion of building and also the remaining 4 dismile land will go. Kindly guide me and let me know is there any chance or point on which I can cancel the agreement with builder ?? What consequences I may face if I go for cancellation ?