He is only
stealing if he
obtained the item outside of legal transaction, and it isn't
stealing if an employee screws the transaction up. If a customer walks out the store having paid for item X - which the OP did, albeit onto a gift card - the customer has done their part, and
the law is on their side. Apple could either cancel the gift card, or bill him - the latter an illegal action, which they would back down if the OP threatened to bring the police into the matter. The store employee is responsible, not the customer.
Jeez, know your consumer rights guys. You must get ripped off buying an ice-cream. It pays to be shop-savvy, and yes he
could call up to note the mistake, but that's a matter of ethics, not legality.