If an ex-employee wants to break a contract, they can do so. But they must return the severance payment.
There is a risk of Specific Performance, but that seems inapplicable here https://en.wikipedia.org/wiki/Specific_performance
that's a very perverse perspective, and is exactly the sentiment a multi-national corporation like IBM would want you to have.
a job is an opportunity for two parties on equal footing to make a favorable exchange: my labor for your cash. it's a classic example of a capitalistic win-win. it's an opportunity for me to ply my trade, to further develop my expertise so that my offerings are more attractive to others and i can make more money. it's an opportunity for you to further your business objectives, widen your offerings, and widen your reach.
slavery is ownership. it's callous and flippant to make such a correlation to work for hire.
Chattel slavery is ownership, but slavery extends beyond just chattel slavery.