Unless you’re some VP/Director/Distinguished Eng/C-level etc, this doesn’t happen in practice. Those jobs don’t follow normal job posting rules anyway, they’re usually sought out.

99.99% of the time, I doubt the job is ever adapted for the person. They come in with that skillset and are usually expected to grow and meet the needs of the other job areas.

So, you want the 1 in 10 000 (likely 1 in 1 000 000) hyperspecific job posting to affect the rules for the rest?

The candidate isn't going to complain if you give them _more_.

Why would you sue for that? There's no contract in place

And in any normal court they will lose since its a different position and they agreed (paper trail) for that switch. Or they could be just let go if company is too careful. Not that such situation happens often in real world, I never even heard about even one.

If they would sue it would also burn their professional creds permanently, nobody sane would ever want to hire such a problematic person.