Did you ask for the Best Rate Guarantee department? Many of us have had that happen. What I've done in the past is gone, paid for the stay, then faxed a copy of the invoice in to the BRG dept. I've had a cheque sent to me for the amount, usually sent out the same day.
I'd not be dealing with the regular customer service people, ask to be connected to the BRG department.
Hope this helps.