If she really wants it then kick up a fuss with manager did you sign any papers stating the car is in your name as of yet?
I think there is a clause stating that if they sell you something at a price, with agreed terms, they are obligated to full fill the good faith of purchsing. The only issue is if you take the deposit back you have no leg to stand on afterwards as you have agreed to terminate the deal?
The only other option is to get the same value or spec for the same money if the car in question doesn't not exist anymore?
Free servicing
Free MOTS
Extended Warrenty
Full road TAX.
Try for these if they will agree?