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.

I remember reading something like this in Marx's first volume of Capital, where he was explaining how the labour is "self-expanding", that is, producing more than it costs to sustain (except he used work-hours in accounting). Funnily enough, if you read Marx's account closely enough, there is no unfairness in payment: what's bought is "ability to work", its marginal price in the competitive market is exactly the cost of living, and the fact that the price of the resulting product the higher is irrelevant.