Signature Global - imposed interest

I have booked a flat in Gurgaon through singature global. After the allocation of flats when my name came under that lottery system, signature global demanded for the first installment with in the due date of **st Sep ****. However, i did not have the money which required me to take loan from bank. Bank does not give loan without builder buyer agreement, i told the same to Signature Global that if you want me to make the payment before that then you should get the BBA done before **st Sep **** (or there must be sufficient time given to customers). however, their customer care representatives told me that "sir, you can take the loan and pay it after **st as it is a genuine case we will waive the interest off. but now they are not doing it and forcibly asking me to clear the imposed interest of *k. I cannot the pay that interest.