Interest asked

I have booked a flat in Aug 2015 and same time I was informed by builder sales team that 90% amount can be financed by the bank as home loan. The lender (PNBHFL) was suggested by them but not in writing. I submitted all the documents for loan processing to the bank. Due date was Sept 26, 2015. Bank informed me that they can finance 85% only and not 90% on Sept 29, 2015 well past the due date of Sept 26, 2015. I lost my 5k processing fee and then applied for home loan with HDFC. Now the builder has charged me interest @12% per annum for the delayed payment for 64 days from the due date and GST @18% on accrued interest. The delay was due to resubmitting all the docs to HDFC and processing from the beginning. my concern 1. Am I liable for the interest amount as the lender was suggested by builder itself and the bank informed me very late past the due date for additional 5% payment as they can finance only 85 %. 2. Is GST applicable on the interest amount, if yes, then whether 18% rate is justified as I am already paying the penalty for delay.