But you can legally structure contracts based on pay ie. Delph earns 60k a week however if he misses set number of games or doesnt meet certain targets i.e. goals/assists etc etc that 60k a week is halved to 30k with the remaining 30k based on pay as you play or performances like many people on commission etc.
Football clubs would love to implement it but the agents run the game and hold all the cards. The fans are also gullible enough to think that players should be paid multi million contracts no matter how often they actually are doing their jobs or to the level required.
Morgan Schneiderlin on 120k a week putting in half arsed performances is sickening but hey ho the fans think thats acceptable so the gravy train keeps on rolling.
You pay your sky or your season tickets then you accept footballers are paid handsomely.
Always have been always will be.