The Sunday Mirror says Ronaldo, 26, a former world player of the year, quit Manchester United for Real in an £80million deal two years ago.
A friend said: "Ronaldo is seriously tempted. He has been gloating that he wants to take the money, join City, score 50 goals in the Premier League and then leave."
The striker lives in Madrid with model girlfriend Irina Shayk, 25, but still has a home near Manchester which he rents out to City's Emmanuel Adebayor.
"He has fond memories of Manchester and still has good friends there," said the friend.