Hi Beata, a proportion of the original advance payment are returned when an early termination has been agreed, this directly comes from motability as a cheque – usually within days of return/exchange of vehicle. As I understand, advance payments are paid each time a qualifying new car is ordered regardless of any dealership.
A loyalty bonus or discount may be considered if ordering from the same dealership, but not paying anymore AP sounds too good to be true.
The part refund can be used towards the Mercedes GLA 180, which is a nice car by the way.