Presumably he can say 'get lost, we have a contract, pay my wages for the next two or three years'?
Could he successfully claim constructive dismissal? If he were out of contract, he'd be able to make more money and they'd be left with no transfer fee.
One of the reasons for ManU picking Barcelona is that they are not a club they are likely to face in the European Cup, unlike eg Real Madrid. Presumably if they tried to include a clause in the sale that said 'you can't sell him onto Real Madrid', it'd be declared an unlawful restraint of trade?