Utter rubbish.
You pay the Advance Payment once you are happy with the vehicle, after you have been shown round it and had any functions you’re not sure of explained and it is the vehicle you want, then and only then do you input your unique PIN (keep that hidden before you are about to use it) and pay any outstanding Advance Payment.
The dealer is either stupid, unsure of procedures or if he is a certified Motability sales rep then he is lying to you, if so it could be for numerous reasons, beware as it has been known for wrong cars to be passed off and once you input your pin you’re stuck with it.
The 1st thing to do is call Motability and explain what the rep has told you and they will contact the dealer and put them straight.
DO NOT GIVE THEM ANY MONEY.