I think at this stage you are jumping the gun. Maybe the customer trying to pay on his phone didn't know how to or had run out of battery and your employee thought it a gesture of good will, albeit misplaced?
At the moment you have evidence of one forgetful moment in relation to non payment of a snack and drink. If it's a regular occurrence you'll capture it on camera in a couple of days and feel a whole lot better about any action you take.
I don't mean to add to your concerns but something you've said rings alarm bells and that's, 'He has been with us for about a year and has always been absolutely great; he often comes in early and does a lot of things above and beyond what is expected.'
My working life started in food retail where petty theft by staff, even eating broken packets of biscuits, was a sackable offence. The real villains though, were smart, often starting early and working late to cover their tracks, making sure they were the one's cashing up the till, tallying the books. In one case the highly thought of butchery manager was tucking sirloin steaks in his socks and taping them to his ankles. He always worked later than normal, offering to lock up as the coast was clear.