If your work is worth $120K to your employer, the difference, like you said, is $20K of value created by the employee.
All this to importantly note that the employment agreement didn't create $120K of value -- the employee did, for the employer.
If the employer takes 2 of you, each individually providing $120,000 of value to their customers, and, through managing you as a team, manages to extract $250,000 from a customer, then the employer has finally created value -- specifically, $10,000 worth.
And anyway, that's not the manager who produced that value: it's the sales team, without them the product is literally worthless. So what is manager paid for, again?
And even more interesting question now would be "what are shareholders are paid for anyway, after they've been repaid the initial investment with the interest on top?"
If your work is worth to me $120K, and you accept $100K, you have created $20K for me.
Therefore our employment agreement created $25K in value each year.