Did he leave it like he was leaving a tip? In that case you can pocket the change.
If they simply forgot it, then I would try and work out who he is, maybe by a store card he has used or whether he is a regular customer... Then, if not, I would just give the money over to the manager who will usually hold it in the store's safe in case he comes back.
If, by Christmas, the customer has not returned, I wouldn't I wouldn't be surprised if the change would go towards some Christmas drinks :)
