I feel pretty certain that they'll just take care of it for you. I don't think you have to worry about legalities at this point.
For some reason charges to my charge cards are as fast as the speed of light. But credits to the same card seem to take longer. On a Hilton overcharge I once had, they told me it would take 10 days to credit my card. And that was about right.
Your case is a little different however and it may take a bit more time for communications between the hotels. But after 3 weeks? I'd say fax it to 'em again and ask for an update.
Hope that helps.
[This message has been edited by PremEx (edited 09-01-2000).]