By handing it in, rather than keeping it, you did the right thing. As to your dilemma, I would call Barclays customer service - they can easily track who withdrew the money based on the details you provide (date, time, location, amount withdrawn). Ask Barclays to keep you informed of progress with their investigation. Finally, call the station manager and tell him/her that Barclays are investigating it and will want the £20 from the person you handed it to (again, date, time and person description is enough I would have thought) - all this will put your mind at ease that the £20 is going to be returned to the rightful owner.