Cant see how this would help in anyway to generate a sale, just a total negative from the off.
If on occasion the light has been on, I complete the deal, then dig a £10 or £20 note out of my own wallet and hand it to them and apologise for not having had a chance to top the fuel up. When they see you are handing your own personal money out it has a more positive effect than saying "Ive just put a tenner in it for you" - all comes from the same pot.