You will report the payback on your 2018 return. Just make sure you get a valid receipt or whatever as proof of payback. That proof will need to stand up in court should it come to that. (I doubt it will, but that's how important it is to get solid undeniable proof of the payback, unless you have no problem with paying taxes on money you don't get to keep.) Without that proof, you didn't pay it back no matter how much you scream, holler, threaten or yell.
