M.G.
Oh like he's baiting you to see if you'll take it.
I don't know if it's legal or not (he'd likely just say it's an oversight if ever pressed on it) but it shows they are testing you. Twice. And you've passed but I get it. What would bother me is if you've shown no reason for them to suspect you would steal, they are just trying to trip you up to see if you'd be tempted.
I would be uneasy too. In all the jobs I've ever managed, I've never set out traps for my staff or set them up to see if they're dishonest.
Awkward because you know they are doing it too. I've not heard of this no .. and I have friends who have been cashiers. Just keep reporting the extra cash but if continues and you grow tired of it, time to find another job where they actually trust you enough to stop testing you.
Good luck :)