In the warehouse scene, the Joker lights the stack of money on fire and says

> 
> Don't worry, I'm only burning my half.
> 
> 
> 

The mafia guy doesn't seem to have much of a reaction to this comment, though (the "my half" part,) it's more that he's shocked about the fact that the money is burning in the first place.

The Joker didn't kill Batman, so why would he have half of the money?