Builder deducting huge amount for cancellation of bookinf for a flat

Dear Sir, I have booked a flat in Pune for 51000 on 4-10-2016 and builder said agreement will done within 1 month, but still no agreement done between us , its over 4 months now, Now decided to cancel booking and builder saying will deduct 50,000 if i cancel how do i proceed further please advise Conditions mentions on booking form as below 1) Agreement will be made within 3-4 weeks upon booking date 2) cancellation charges is 50,000 post booking