The dealership could easily discount to the original order price if they wanted to as a goodwill gesture. If not then it’s either you buy it at the AP now or walk away and wait for another car.
Not very helpful but your options are limited.
When you order a vehicle it goes through the normal dealer system and separate motability system to approve the car the dealer is ordering, so it just sounds like the old salesman put it through BMW system to meet his targets but didn’t care enough to put it through Motability because he knew he was leaving and it won’t be his mess to clear up.