With all due respect, a few people have said/suggested this on the thread, but what's wrong with that really?
Were I a footballer, I'd love to play for United, but I wouldn't necessarily do so if another club of comparable size/status offered me more money.
As Roy Keane said (in a different context) "it's his job" - plenty of people would take a higher paid job than a lower paid one, plenty would have other considerations. Horses for courses and all that.
I'd love him back, as he'd be a fantastic player still at our level, but I can't see Burnley letting him go.