I think you DO have reason to get rid of this person - as Squitty and buildersmate've said.
I'd have this person into the ofice and ask him to justify his claim for expenses - correcting him where he's wrong. However, he could plead that as nothing's been said before now, he "thought it was alright" - along with other excuses. I then think this might be the time to sack him for gross misconduct, saying that he's welcome to go to a tribunal, but with the evidence to hand, it'd be a waste of time.