I did not realize someone could get a scammer tag even if they attempted to scam someone else. I always thought the scam had to be committed and someone lost bitcoins or some other valuable resource.
No, you're right, scammer tags aren't given if no scam is committed. If his allegations are true, only thing Usagi lost is time, and that isn't worth a scammer tag.
It's also not likely anyone will receive a scammer tag for not returning the double payments Nefario made. If only because doing so would require trusting Nefario and his data, and he's already shown himself untrustworthy.