If I was the buyer, I would not be too happy. My first thought was that you're just trying to squeeze more money out of me since I agreed to take the deal.
Maybe one way to have a happy ending would be to tell the buyer you screwed up, the price should have been $500 more than you asked, and you'd be willing to give him $25 as a "cancellation fee" to cancel the deal. Or, you'd be willing to go through with the deal for $400 more. That way, the buyer still gets a great bargain, you only have to eat $100 loss for the mistake, and the buyer doesn't feel you are trying to squeeze him, since you willingly offered him $25 for the inconvenience.