The capitalist answer is they don't have any right to your labor.

They purchase your labor with the wages that they pay you. You are free to not accept any opportunity offered, and either party can terminate it at any time for any (legal) reason if they feel the deal is no longer to their satisfaction.

Almost everyone believes that they are underpaid. There is a very simple way to prove it by checking your worth in the marketplace.

You are also free to ply your trade on your own if you so wish.