There should only be vapour in the tank, not much more.
A few months ago we suffered a catastrophic fuel leak, losing 50+ litres literally in seconds, the car was completely drained
When the dealer called to say the car has been repaired I asked if they’d put any fuel in it, and the dealer replied yes
When I picked it up, they’d literally put a litre of diesel in it. I set off for the nearest station and the range clicked to zero before I even got there, engine was still running as I pulled up though.
No matter how friendly the dealer seems, you do not owe them anything. Don’t stick more fuel in than necessary, they won’t thank you for doing it or think badly of you if you don’t. There’s no etiquette to observe, as long as they can get the car on to a transporter to ship to British Car Auctions (or wherever Motability vehicles go to die) then they do not care.
The only people who would benefit are the owners of the petrol station and maybe the next recipient of the vehicle.