Would an honest researcher actually have to pay back their salary? Recently, I read the answer by Hexal to the following question here: Not including student who contributed very little as coauthor of paper and I was scared. Much more than scared, I could not sleep the following night. In the question, a project leader asks whether or not he should include a visiting student who showed &quot;good motivation&quot;, but did not contribute anything to a certain project as a coauthor (to one of the project's papers).
There is a downvoted answer with -40, saying that if the student writes that she wants to be a coauthor, the project leader should write her an e-mail:

[...] Therefore, we would appreciate it if you return the salary you received for the internship. Please transfer USD ... to the account ... until .... If we don't see a reverse payment, we feel necessitated to undertake legal steps. Thank you for your cooperation.

Now, this answer is downvoted. I am also not sure if the downvotes/comments are because this approach is not appropriate for the given situation or not appropriate in general.
This scared me a lot. I always thought that one would never have to pay back earned money (except for extreme reasons, like fraud probably?).
Now, I do not plan to not work. But I know some asshole professors who, when their students say that they do something else on their weekends than working, they tell the whole faculty that their student is not working. I've never heard of someone asking back for money, though. However, I am scared now that I will work for such a professor, we do not manage to write a paper together and then they make me to pay my salaries back after some time, which I cannot afford.
So, my questions:

Are such threats something that happens sometimes / often?

(I would probably be scared enough if I got such a letter since lawyers are expensive but) Did students ever go to court because of such a letter and do judges approve this?

Are things like this supported by universities or their legal departments?


I would be interested in answers about the situation all over the world – if this is not possible, I would be interested in Europe and Northern America.
No, that answer is ridiculous. 

In North America, as long as you were working honestly*, the only reason you would ever have to return money is if you were accidentally overpaid (i.e. they paid you more money than you earned).

* I assume you aren't actually trying to commit fraud