Early on, Eddie gets arrested.
Eddie then makes a deal with the cops to set up John.
Eddie then agrees to sell John $18K worth of coke.
After getting arrested and going to court, it is revealed that Eddie sold him mannitol rather than coke.
So since buying mannitol wouldn't stick as a crime, why bother? Clearly the cops would not give Eddie 'credit' for this?
Whose idea was it for the word "Lisp" to have an "S" in it?
Because the cops didn't know it was Mannitol at the time of arrest and John's attorney (David), would be the person privy to the toxicology analysis besides the prosecution.
That's why the prosecution agrees to drop the charges and John questions David about not selling him cocaine. This is also why David tells John that had he didn't, he'd be in jail.
So for John keeping his mouth shut, he gets in good with David and Barbosa, Eddie gets killed for being a rat and the movie goes on.