Indian Railways — persons in civil dress collecting unauthorised charges for penalty

Samparkkranti train, 12649, where due to festive season their is over occupancy of commuters. I'd to urgently go to nagpur so i'd to buy a general ticket worth rs 300 from yesvantpur to nagpur. Since their was no space left even to stand in general coaches so i'd to board a sleeper coach. I told tte to upgrade the ticket but he said it's not possible. Suddenly at 1am these persons came and charged me extra rs 500 for nagpur, telling me it's penalty. I said the charge for sleeper coach to nagpur is rs 540 and you are taking extra. He was extremely rude and forced me to pay extra rs 500 even after i was trying to make him understand. By this incident we can only say that it's like extracting money from helpless commuters. For a sleeper ticket of rs 540 for nagpur i'd to pay rs 800! And still i'm not having a seat! I'm a student and money doesn't come easily to pay for these exorbitant charges. It should be reasonable.