Interesting! I've never seen this, but they aren't required to honor it. Hopefully they will as a goodwill gesture. As noted above, definitely have a back-up reservation and don't throw too much of a fit if they don't honor it. I would do the pay now option, if possible, to have more leverage against them.

Care to share any details on the rental (approx date, location) so we can try to find the score? Or if it's on the east coast, just PM me and I'll see if I can make use of it if you don't want to post publicly.