Charged falsely from private car rental company

This is to bring to your notice that on 28th April, 2018 I picked up a Ford Aspire car from Zoomcar which was handed over to me by their executive at Noida (Opposite GIP mall) at around 9:30 pm. I also returned the car perfectly without any damage on 29th April, 2018 at 03.21 am. There was no issues at that time or even after a few days. After 2 days i.e. (1st may 12:05am), I received a sms informing me that I have been charged Rs. 10000 extra on account of vehicle damage fees. This surprised me as the executive who took back the car said that the car was perfectly ok and he did not inform me about any damage etc. Even i received a mail on 29th apr at 11:06 am that there's no outstanding amount due ,then i mailed their customer care to discuss about the matter. At this point of time i did not receive any single call from their side about such fees. When i discussed with customer care personnel, they informed me that fees charged is valid and this need to be given at once or else zoom car will take legal action against me, and yesterday i received a threatening legal notice from zoomcar, I want to know the procedure of defamation because they mentally harrassing me since 1 month Thanks, Zoheb Ali Khan