Suppose someone ordered supplies totaling over $1,000. When he picked up the purchase, he was issued a refund that the order was canceled. He now has all the products he ordered and was not charged. This was not due to him canceling the order; the employee did so wrongfully.
Ignoring morals, is there legal obligation for him to attempt to return the refund? Does this fall under unjust enrichment? If so, to what extent must he try?