Pretty tough to prove it absolutely.
You could try to show Masiello never approached Bonucci. The lawyers are trying to do this by pointing out how Masiello keeps changing his version of how he approached Bonucci (including presenting an alibi for one of the times Masiello claimed to have spoken to Bonucci), to suggest Masiello just made the whole thing up.
You could try to show there was never a fix. The Lawyers are trying to do this by highlighting the conflicting versions of what the fix was meant to be.
You can try to show there is no evidence of Bonucci ever recieved any money from Masiello. Again, they lawyers are trying to do this. Afterall, if Bonucci was going to get a cut (8,000 if I remember correctly) from the fix, surely Bonucci would have received that in the days/weeks after the fix.
Beyond that you are just reducing it to one man's word against another's and trying to show your guy is to be trusted while the other isn't.
It's another case where I can't see how this charge could be made to stick in a real court, but it is very tough when the onus is on the defence to prove innocent.