Give them a call and explain your situation. They are usually very understanding and helpful.
When my wife couldn’t drive any more we no longer needed two cars. As my private car was significantly better than anything available through Motability, I called them to discuss returning our Motability car. They were fine about it.
From memory, we would have received a pro-rata refund of the AP, less an early termination fee which I think was around £250. There was no pro rata refund of any options we had added. I thought that this was very reasonable but, in the end, I sold the private car as my wife found it harder to get in and out of and the Motability car was also easier to load the wheelchair in and out of.