The funny thing... (spoilers)

...is not what the black guy did, but what the woman said afterwards and how she said it, her choice of words. Furthermore, what the black guy did could only have worked with a woman as racist as the one depicted in the movie. If he had jokingly swallowed the ticket of a regular old woman, she might have told the tram ticket inspector "The guy next to me just ate my ticket!" And the inspector, knowing that some people are as stupid as to mock an old lady, might have believed her, allthough her excuse for not having a ticket was a bit far out.
But the racist woman said: "Der Neger hier hat ihn (Fahrschein) eben aufgefressen." rough engl. translation: "This ni...ger has just eaten my ticket." Allthough "eaten" is a much to weak translation for the German "fressen", because "fressen" means "eat like an animal."
And by using the word "fressen" and ni...ger she outet herself as a racist and it was clear for the inspector, that the old woman could only have lied.