In economic terms, an agreement can happen where both parties extract value from the arrangement. A fair agreement is where both parties extract proportionally similar value. An unfair agreement is when one party has few alternatives, so the other party can negotiate terms such as to capture most of the value.
If you deal fairly with people, you will offer them agreements that provide them with proportional value. You will do so even when you could negotiate them into terms that are much better for you than for them.
Walmart could be considered an entity that extracts all the value in their agreements with employees, because the counterparty has very limited other options. These are consensual agreements, but they aren't fair. Making such agreements is legal; but that doesn't get you off the hook as a scumbag.
Many countries have legal requirements which attempt to ensure that certain types of agreements - especially in employment - are not only consensual, but fair. This is a form of state coercion which I've come to think is necessary, and welcome. Employment is an area where allowing people to be scumbags has especially detrimental results.
But employment isn't the only area where an agreement, or a mutual promise, can be unfair.