At Eurolines ticket seller, I paid 10.000HUF for the fare of 6900HUF and the female clerk returned me 2 1000HUF notes and some coins. At that time, I already put the money in my wallet and could not consider about checking the fare on the ticket. So, I did not ask about it to the female clerk there. There were two men - one in German security Internet address logo jacket talking to the clerk about tomorrow's ticket. I don't know if they worked together for the money theft. But I think I should be careful in Budapest for handling money.
I found some red letter German cars around in Budapest. What are they? I must ask German TIs for the clue.
And here is the video.