Could it even be theft now maybe. He has goods in possession that he knows belongs to someone else. maybe not theft originally as no intention at the time but now the intention is there. This would be a criminal offence - if not theft something similar (I am not a criminal lawyer).
Very, very technically theft. But realistically the police (at least the ones I deal with) would never follow up this sort of thing.
Its the fraud that might actually have legs because the shop is already starting to allege it, and Julian's defence of not realising at the time gets much less credible if he decides he wants to delay paying the money once he's been notified by the store.
Hence my advice of paying the money asap. Get apologies later.