If someone pays you $100K, then your value is, by definition, $100K, because the value is what someone is willing to pay. 100% of that value, by the way, is created by you developing from a $0K-salary baby into a $100K-salary adult. So that's 100K value created by the employee (and the people in their life to an immeasurable degree).

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.