Appearance of money on film isn't usually a legal problem for the production company, though. The filmmakers and the prop owners don't necessarily have the same priorities - if it cost the film more to rent authentic-looking cash for whatever reason, that's just a line item in the budget. It can even be worth hiring the real cash at a premium because the armored truck, hawk-eyed security guards, on-set audits etc. may be worth it in marketing terms.
Sure. Though many of the things needing reflection would also be props, and if the Secret Service were sufficiently interested in cracking down that mirroring or real cash were the only real options, mirroring would itself be "just a line item" to be compared with the other. Like I said, though, probably not actually practical.
Color correction, on the other hand, might be a practical approach...
I can definitely see potential in the color correction idea, but usually that involves keying ona fairly narrow range of color so I don't know how it would hold up under a wide variety of lighting conditions - although it's not like you're scattering money in every scene unless it's a very very bad heist movie XD
I have a colorist friend that does a lot of mega-budget projects so I'll drop him a line and ask...although I suspect his answer will be 'anything's possible if you're willing to write a large enough check.'
