This is just like vouchers and gift tokens.
If you tried to return it because you simply have changed your mind then I would say you would have to accept the credit back onto the card if that's what they insisted on. However, because the item is faulty and they are in breach of contract then I think you are entitled to the cash.
Did you complain to the dealership about only having one key back in 2017 when you collected it? If not and this is the first time you've said anything about it to the dealer I can't think how you could ever prove your argument. So yes you probably will have to suck it up and buy a new key. The moral of the story is to complain at the time.
Maybe they can't prove they gave you two keys, but on the other hand you can't prove they didn't and two keys is what's the standard provision by the manufacturer. Go through all the documentation you got when you collected it. Does it mention keys anywhere?
I bought a new car on PCP from a BMW dealership in 2017.
I was only given one key when I went to collect it.
The dealership is adamant they give two keys with every new car but have nothing to prove they did.
BMW FS want to see two keys when I hand it back;
I've never seen two keys for the car!
I've got a complaint open with the dealership, but what are my options?
Am I going to have to suck it up and buy a new key for a car that I only got one for?