Today I watched American Gangster. The story about Frank Lucas.

!*Spoiler*!

In the end Frank works together with Richie. They make a deal. He could choose between 'rich and in prison, or poor outside of prison'. Therefore he has to point out the corrupt policemen. That he does. So his part of the deal is over.

Then the movie is almost over and the history is written in the movie (can't explain it otherwise..)...Frank Lucas got 70 years of prison (later 15)...but that wasn't the deal with Richie.

My question: how does the system of crown witness works here?