In Alex Ferguson: My Autobiography, which goes on general sale on Thursday, the Scot tells of how then Madrid president Ramon Calderon's public overtures riled him to the extent that he refused to sanction a sale despite Ronaldo's desire to move to Spain.
Ronaldo would eventually join Madrid for £80 million (€94m) in 2009, which Ferguson believes was a statement of intent from re-instated president Florentino Perez.
The Scot writes: "Madrid paid £80 million in cash for him, and do you know why? It was a way for Florentino Perez, their president, to say to the world, ‘We are Real Madrid, we are the biggest of the lot'.
"When the possibility of him leaving edged towards being a probability, I reached a gentleman’s agreement with him.
