Builder is forcing for payment

I have booked property 3 months ago its ready position new flat and done agreement registered I have done yet the payment of one lack by check and put the file for loan but now builder sent me letter that I have to pay the bal amount in 15 days with the interest of 1 lack the bal amount is 23 lack now what I can do please help im not in the position to pay im helpless the bank is saying 1 more month and builder did not said anything about interest at the time of booking that if loan from bank takes time now suddenly I have got this letter is this is legal cause the property was ready position at the time of booking is there no law for the person who is going to take loan he will not get time for loan approvals please help