Why pay more than you need to?
The club must have had signals that he wasnt happy and might want to the move. An offer was put in and rejected, as is usually the case. If the player is agitiated and wants to go then that stregthens your negotiating position.
What some people seemingly fail to realise is that negotiations will "unoffically" start long before a bid is made so that the club can decide if there's any point. What would clearly have happened her is that if Fabregas had asked Barca to move, then they would have probably accepted that and told United what they want.
Why make a mega bid for a player when if he's unhappy and forces the selling clubs hand you might get him for much much less? The key is whether the player wants the move, if he does you then seriously discuss what the club wants for the player. You can discuss the move with the players agent before so why make a huge bid first?