[spoiler]In 1897 Ang tells Barnabas that she has to get the love of a mortal man in order to stay out of hell and live a normal life, which is what she wants. But, the catch is that she can't use magic to get him.[/spoiler]
Now I can't remember if she said that she can't use it to directly influence him, or she just can't use it at all. If it is the latter, then wouldn't she have been sent to the firey pits a long flipping time ago?
She used TONS of magic to get Barnabas to try to come to her--Making Joshua disappear, putting Vicki in danger with Trask, driving Josette to Jeremiah, etc etc. Not to mention that she used magic as a blackmailing force to get Quentin to let Amanda Harris go. Hmm. Seems fishy to me.
Those are two separate stories. I don't think she had that arrangement with Diabolos in 1795. I think her being allowed to return had that deal as part of it and that was in 1897. So, while she used plenty of magic in 1795, in order to fulfill the "deal" in 1897, she wasn't supposed to use it.