Airtel — charged for free data

Website:

Airtel

I have airtel postpaid plan having A/C no:[protected] has been offered free 10 GB data .in the bill month of 2nd May to 1st June has been charged with Rs.507 for data usage.I had visited Airtel center twice and they have lodged my complaint regarding unfair charges and given assurance to revert back of charges .In my aitel app it is still showing 78Gb data available.
i would like to know if my free data validity expired then how it is being reflected in my account.it is unfair to charge without informing about expiring of data or whatever.
your customer care denied to revert it back by calling back on same day after my complaint.
Please do needful otherwise i have to think about another service provider.
I am attaching here a screen shot of my accumulated free data in My Airtel App
email: [protected]@gmail.com
Relationship No:[protected]
Bill No:[protected]