I faced this as a foreigner working as a post-doc at a French company where it turns out French labor laws are a double-edged sword. I was offered a much better job towards the end of my contract and therefore handed my resignation in. I was then summoned to a meeting by HR and told if I wanted to leave before the end of my set-term contract (CDD) I was required to pay a considerable amount of money.

Thankfully my own manager (whom I was leaving) had my back and pointed out that since I was leaving for a permanent role (CDI) which my current employer had not yet offered to me in writing (though informally expected me to carry on), I could get around this.

I honestly don't know what they were expecting. How can you carry on working for a company against your will with the threat of being sued if you tried to leave, what sort of a working environment is that?