Ronaldo's future has come under increasing speculation in recent months, with the 32-year-old understood to be close to leaving the club last summer following allegations of tax evasion by the Spanish authorities.
The 32-year-old is also thought to have grown disillusioned with the team's form on the field, with this weekend's 1-0 defeat at home to Villarreal leaving them closer to the La Liga relegation zone than leaders and arch-rivals Barcelona.
Spanish publication AS now reports that Madrid president Florentino Perez has failed to deliver on the promise of a pay rise for Ronaldo, despite the Portuguese winning his fifth Ballon d'Or in 2017.
Ronaldo has slipped down to fifth on the list of football's highest earners and had been expecting an improved deal at the Bernabeu following his Ballon d'Or triumph.
Madrid's failure to meet their promise has reportedly increased Ronaldo's desire to leave the capital, with a return to Old Trafford - where he spent six years from 2003 to 2009 - one of his preferred destinations.