It’s a 5 minute job for the dealer to cancel the ‘order’ on the Motability system and resubmit at the lower AP. You would still receive the same car, as the Motability system is just an admin thing, not the actual vehicle order. Many of us have done this via helpful dealers (my BMW dealer did it without me even asking).
However, if your car is within days of being delivered, you may find that your dealer is unwilling, for fear that they may get some sanction from Motability (although this is extremely unlikely). I believe that Volvo dealers aren’t the most helpful out there. I would certainly ask and hopefully they will oblige but, if they won’t, with that level of price drop I’d be tempted to cancel and re-order. You are quite at liberty to do this at any stage prior to entering your PIN on collection day. Unless the dealer is bloody minded, you would probably still get the same car! Let us know how you get on.