You have every right to deny a sale that hasn't actually taken place (no payment) - with or without explanation. Even if the explanation is that it has simply become too much of a hassle, so you're going to keep her for now. I know "things come up", and I like to think I'm very flexible, but I would just stop contacting her and look elsewhere. If she makes good before you find another home, great; if not, it was her delay that caused the loss.