| | Re: rhetorical questions (with question tags)
In conversation at least, it feels like you can phrase a tag question in positive or negative, if the first part is positive, but not if the first part is negative. I'm not sure whether technically you should, phrase it positive positive, but there are probably plenty of examples of this in movie scripts or everyday conversation...
i.e. with reference to your example:
He paid you back, didn't he?, or He didn't pay you back, did he? (are the normal ways to phrase this question)
"He paid you back, did he? " Seems ok to my ears. It pretty much means the same thing, but has a different shade of meaning (it sounds very casual, and may be technically incorrect, so you are safest to avoid using that, especially in tests).
"He didn't pay you back, didn't he?" Just sounds wrong.
Hope that helps.