I don't think Fergie would ever willingly sell a player with any value at all to a Premiership 'rival'. Has he done it before? I'm struggling to think of an occasion, though in sure someone will remember one. Berbatov maybe, though Fulham are barely in the same half of the table.
Anyway, much as I dislike Nani, his selfishness & arrogance, I'm warming a little to the idea, given some of the other options we've been linked with.