At the ending of Man on a Ledge, Nick Casidy is able to prove his innocence by showing publicly that the bad guy still has the diamond in his possession, which proves that Nick never stole it.
This only happens though after the whole plan goes completely wrong. Initially Nick's brother was supposed to steal the diamond and Nick was supposed to get away with it in his jacket. That's when he got caught.
At that point, how would he reasonably prove that he did not steal the diamond 2 years back but just that day?
Just having the diamond only proves that he stole it after all. What was the original plan before it went wrong?