Non payment of interest to builder

Hi, I booked my flat on Aug, 2016.As per their agreement 20% amount to be paid at the time of booking but i didn't paid as was waiting for bank to check property is valid/approved or not so i made payment late and due to that the next 2 payment also got delayed because i took loan from SBI which takes time to process loan and disbursements. Now after 1.5 yr later builder is asking interest for late payment. I told them that due to bank loan approval and process it took time and got delayed. They said that they provide 20 days(which is mentioned in agreement letter also) of time and after 20 days they calculate interest @rate of 11%. Also they sent demand letter through email and from that date they start calculate 20 days and later they sent original hard-copy of letter and my sbi do not accept letter through email. How can i avoid this interest part as its too much for me. Please suggest me is there any way or rule to fight with them.