a. It appears that you have already paid 70% of the total consideration including the bank disbursement which stands as loan in your name. So. the builder can easily deduct 10% of the total sale price and refund the balance amount of 60% to HFC while cancelling the said flat.
b. You can inform the HFC about the said cancellation but please note the you shall stand as a defaulting before the HFC liable to repay your loan and your CIBIL score will nosedive.
c. You shall stand as willful defaulter before the HFC who can file a suit against you claiming refund of the said balance 5% and interest thereupon for which the Court may pass order for attaching your other properties.