However, the rules do not specifically state that the club that owns the player has to give consent.
United's initial complaint said: "On 27 May, United made clear its intention to report Real Madrid if it continued to involve itself in the future of Ronaldo.
"Unfortunately, Real Madrid has not kept its own counsel."
Real coach Bernd Schuster claimed Ronaldo was their main summer transfer target, triggering United's initial threat of a complaint, while United boss Sir Alex Ferguson has accused the Spanish club of a "lack of morals" for allegedly unsettling the player.
At the start of June, Ronaldo reported told Brazilia website Terra he wanted to play at Madrid, "but only if it's true they are ready to pay what Manchester United ask of them".
In response Calderon told Spanish newspaper AS: "For Madrid it is an honour to know that a player like him thinks that playing at Real would be good. We are proud to know that he would like to be with us."
Ronaldo, currently with Portugal at Euro 2008, has said he will not speak about the subject again before the end of the tournament but he did say he had yet to decide about his future.
Despite being under contract at Old Trafford until 2012, Ronaldo has refused to state publicly and unequivocally his future lies with United.
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