Spanish publication AS says that Old Trafford officials already "know his desire" to return to Jose Mourinho's side with the Portugal international having informed some Real teammates about his "firm" decision to leave the Santiago Bernabeu.
AS claims that Perez promised Ronaldo a pay rise but seven months later the Real president is yet to offer him a new deal.
Ronaldo is currently earning €21m (£18.7m, $25.7m) per season – equivalent to £360,000-a-week - after he signed a new deal until 2021 in November 2016.
And Ronaldo is said to have made the decision to leave the Santiago Bernabeu and return to United.
Ronaldo won the Champions League, three Premier League titles, one FA Cup and two League Cups with United before completing a then world-record move to Real in the summer of 2009.
Ronaldo has publicly stated many times his love for United remains and AS claim that the player now believes that Old Trafford would be the "ideal" place to continue his career.
Last summer transfer window reports from Portugal claimed that Ronaldo wanted to leave Real after being upset by allegations that he knowingly failed to pay up to £13m in taxes to the Spanish authorities.
The player appeared later in court to deny those allegations but added that he would like to return to United after lamenting that he never had any problems during his playing days in the Premier League.
"I never had these problems in England. That's why I want to go back there," Ronaldo was heard on camera saying during his appearance on court.
Ronaldo remained with Real for another season but he has since endured a difficult start to the campaign.
He has only scored four goals in La Liga this term with Los Blancos saying all but goodbye to their hopes of retaining the title, having fallen 19 points behind Barcelona after 19 games.