As an auditor, I would suggest that in lieu of sticking a larger bill in the machine, you use a redeemable card or certificate (I'm not an ATM technician, so I have no idea how to do it). The customer would bring in the card or certificate to receive their cash. That would provide an audit trail to ensure that the bonuses actually go to customers, not the ATM teller.
Also, if I were a customer of the institution, I would withdraw the maximum for the day in order to increase my odds of winning, then pull through the drive through and re-deposit what I didn't need. Have fun re-supplying that ATM every day. Not to mention forcing your teller to sell to the vault. Do enough customers not get the same idea for this to cause a problem?