He doesn't technically receive the money in a transaction sense.
His receipt of the money is 0 in actuality.
In the future should he kill Batman, he would receive 50%.
However at that point he had 100% of the money, one guy opposing him against his ever growing army of psychos, what do they care about money? What do they care if they rightfully received the money or not. The whole point of Joker getting "paid" to kill batman was just a ploy anyway to turn the mob upside down, the city upside down, and batman upside down anyway.
The Joker doesn't care about money. He doesn't care about fame. He doesn't care about life. He only cares about chaos.
Shortly after this act he states:
It's not about the money, its about sending a message
As his goons escort the mobster off to his demise. The Joker merely brought him there to show that he retrieved Lau and his money, only to shortly show the mobster how
All you care about is money. This city deserves a better class of criminal. And I'm gonna give it to them!
Joker doesn't want to kill Batman, he has too much fun with him, trying to show him how the natural order of the world is chaos.
You won't kill me out of some misplaced sense of self-righteousness, and I won't kill you because you're too much fun.
It can be argued that he knew of this fact long before he made his offer to the mobsters to kill Batman for 50% of their money. They were just another pawn in his long game for the soul of Gotham.
To clarify some points about the scene. All of the people there besides the mobster are under Joker's command. After escaping the police station with Lau, he most likely forced Lau to show him where he had stashed the money, which is most likely where they were to begin with. He then tied up Lau and contacted the mobster to retrieve his money from him. At that point the scene commences, Joker burns the money (and Lau) and has the mobster taken away to be fed to his own dogs.
At no point was the Joker actually "paid" for his services. He merely burns all of the money as a display of his wanton disregard for the value of currency.