Not sure if your definition is right! For many of us working for a wage from time to time are
Forced to do work for Employers (against our will if you like, because if our will prevails, we would rather be sitting around watching TV instead of working for Employers!)
But do note the definition in Wiki:
Slavery (also called thralldom) is a social-economic system under which certain persons — known as slaves — are deprived of personal freedom and compelled to work.
Slaves are held against their will from the time of their capture, purchase, or birth, and are
deprived of the right to leave, to refuse to work,
or to receive compensation (such as wages) in return for their labor. As such, slavery is one form of unfree labor.
Obviously the wage element is there somewhere for one to defy what 'slave' is!!!!
Ronaldo was only HELD at UNITED cos he has a contractual obligation to meet, and is NEVER deprived of the right to leave arising out of a slavery sense! Get over it!