Good morning Alison and welcome to the forum.
You obviously took your dealer at his/her word and arranged to sell your car in preparation for the delivery so I fully sympathise with you.
My first port of call would be to call Motability and explain your situation. Perhaps they may be able to suggest something. And an important thing to remember is that you can cancel your order at any point up until putting your Motability PIN number in at the dealers when picking the car up.
Sadly, you have found out the hard way but I always try to abide by the three rules of dealing with car salesman, that is generally:
1) Don’t believe them
2) Don’t believe them and,
3) Don’t believe them
Best of luck. ?
? I will be remembered for nothing but had great fun doing it ?