I doubt the club would agree to basing salary elements on number of goals; maybe a more complicated metric based on a combo of metrics including, eg, score involvements.
I can't see there being overly complicated trigger, the player has to agree with it. It'd be less about governing behavior and more about providing an clear and achievable threshold. They'd also want to be able to balance the various triggers of different players the simpler the triggers are the easier it is to manage.