And all of that is conflated by the fact that the person is already hired so it isn't a hiring question, it is a pay question. And at least in the several "managing legally" I've been to, paying people differently doesn't get you into legal trouble like not hiring or firing them does.
That said, at one of the companies where I worked there was a suggestion for engineers to go to India to help train local engineers, and having their salary be the same as their peers would have in India. (as opposed to California salaries) It was controversial to say the least.