I think that they probably had an obligation to get the tank read just prior to closing. If you haven't filled it since then, or you have and know the approximate balance at the time of closing, you might want to keep them as a friend and offer something. You never know when you might need their help with some other aspect of the home that they would know about since they lived in it before.
Your contract however may specify what happens in this event. I'd start there and see exactly what the language says. Since you agreed to it up front, they could make a claim. Your defense may be that you weren't given proper or reliable notice, however a mediator or judge may still feel you owe something.
Work this out on your own if possible.... more