Big Bazaar / Futurebazaar / Future Retail / Hometown — charged more than the mrp printed on product

Big bazaar at ampa skywalk mall, nm road charged me rs.100 for cadbury dark chocolate, but the mrp on the product is mentioned rs.90. I checked the same after i paid and received my bill from counter. Customer representative available inside their store offered me to refund the excess collected money, but i want to ask big bazaar, what about those customers who didn’t check the bill and you cheated them? I refused to take cash refund as i paid in card and i don’t know how many such items i have purchased from bog bazaar were charged excess than mrp!
Kindly explain to the issue and arrange for suitable compensation at the earliest! Bill & product picture captured inside store is attached.