I think both of those ideas are wrong. A company is usually entitled to hire the most productive employees it can find for the remuneration it’s willing to offe

I think both of those ideas are wrong. A company is usually entitled to hire the most productive employees it can find for the remuneration it’s willing to offer, and an employee is usually entitled to somewhere between 50-90% of the money it can convince an employer to pay them. Employees are never paid for the marginal value they provide, because for among other reasons, there’s no such thing. You could come up with an infinite number of different ways to calculate that, which could all produce a different value. An employee is either paid something close to the equilibrium price for the labour they’re offering, or something close to the average equilibrium price of all employed union members, if they’re a union member.