Wow, astonishing.
Non refundable deposits are taken for things that cost people money when they are bought, i.e a loss to the business.
Why are you trying to charge the guy 100quid for not having lost a penny.
That aside, it would make you a decent human being, giving him his money back. He might really need it, again without you losing out.
Sounds like your trying to make a very quick very dishonest buck imo by even attempting to ask your "rights".
Unless the station was a substantial distance away and you lost out, give him his money back.
Shame.
ps... Any bank reader from any bank works with any card. I'm making the assumption you have one too, since I have 3 in this house. You could have sold it there and then with that piece of info.