Irctc — taking extra charge for one liter of water

I was traveling from thiruananthapuram to Changanacherry on manglure express 06402 .when it reached kollam at 10 .10pm I bought 1liter of water and gave him 100rs he gave 80 rs back, I enquired about the mrp he told me that its 20rs but I found out that it is 15 rs he was not ready to give the balance . he and his colleague's weredoing the same .please take necessary action so that this should not happen again