The worst plot hole

So, I see there are plenty of threads about the plot holes in this movie, but not the biggest one:

How does showing the diamond at the end prove his innocence?

Remember, he's been convicted of stealing it.
The only proof he now has that he never did it, is the fact that he appeared to pull it from Englander's pocket.
Well, I can make it look like I found a coin behind a child's ear.

All Englander has to say is "Fantastic! I guess you didn't break it up and/or sell it after all". Cassidy gets time off his sentence maybe but is not exonerated.

(You could argue that the crooked cop going crazy, and the growing Internal Affairs evidence is enough to have his conviction quashed. That may be true, but that evidence would have to largely stand on its own. The diamond reveal doesn't help that much).