You generally don't want to do this, because it perverts the incentives. When you offer cash for something that should be done out of some sense of morality, you turn it from a terminal value ("I do this because I want it to be done") into an instrumental value ("I don't care about this but I do it because I want the reward"). You'll start getting much more noise with the signal because of various "entrepreneurs" who smell the occasion for a quick buck. There's ton of psychological/sociological research which shows that people generally get funny in the head when you substitute social incentives for economic ones.
How do you know they're not leaking it for the cash?
Why do we care, so long as the leak is accurate and - therefore - public well-being is achieved?
Because the leak may not end up being really accurate; when the contribution is done for cash and not out of care for public well-being, then all bets are off.