Here's an idea about what Xander's secret could be (it comes from someone I had a chat with on Buffytube). Maybe he's been lying to Dawn that he's earning more money than he really is, because he wants to provide for her education, and he got himself in a debt. That would be a real life problem, the kind that season 9 is supposed to be about. And it would be in character, he always worried if he could be successful enough.
Buffy may or may not know. But maybe she doesn't know, and Dawn and Xander helped her out with some money, to help her get the new apartment. And that's an additional reason Xander feels some resentment at her while looking at her nice new place.