As revealed by Goal on Wednesday, Spurs have opened talks with the Spanish giants over a potential £100 million deal.
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"You never know but, when the player is willing to leave, it is not easy to keep him when he wants to go," the former Blancos chief told Sky Sports News.
"Spurs don't want the player to go but it seems that the player has decided to leave, so it will be difficult.
Calderon witnessed the loss of Robinho to Manchester City during his tenure at the club once the Brazilian forward expressed his determination to move to England and says that Spurs must now maximise their potential profit.
"We had the same with Robinho when he wanted to leave in 2009. After that, you just have to try and get as much money as you can," he recalled.
"The figure being talked about is a huge amount of money."