chances are, customer gives him cash straight in hand while he's on the shop floor, customer walks off, he goes over to till 5 minutes later to ring it in etc, rings it in as 80 and chucks the reciept in the bin. i've certainly dealt with customers who just want to put the money in your hand and get out of the building
and yes it's a lot easier to steal the money if you're the manager etc, noone really suspects you if they see you just jotting things down, till drawer open or whatnot. this guy is just a student working the odd day a week, he's no responsability, he couldn't give a f*** about the company. when i get new employee's in, more often or not theiyre cheap stuednt who do a couple of days a week, if they treat it as just another few pints at the end of the week etc, i'll have them straight out of the door
and yes, it's all assumption. all i can tell for sure is he's a student, so young, and it's a temp job, so he'll be nothing more than a shop salesperson or tillworker etc.