The Red Devils, along with the Portuguese superstar's first club Sporting Lisbon, were promised a percentage of any future sale above the £85 million fee the Manchester giants were originally paid by Real Madrid back in 2009.
Ronaldo will reportedly pick up wages of £500,000-per-week in Turin, after claims United had turned down an approach from his agent Jorge Mendes to take the five-time Ballon d'Or winner back to Old Trafford.
United boss Jose Mourinho felt the money would be better used strengthening a number of key areas in his squad, although people at the club felt Ronaldo had set his heart on playing for the Italian giants for a while.
United have spent around £75 million so far this summer and have recouped very little in sales, with Mourinho still hoping to get another three faces through the door before the transfer window closes next month.
The Ronaldo windfall might not be a huge amount in the larger scheme of things, but the money will be added to the £100 million pot Jose still has available to spend.