Never bothered to sign up before, but I wanted to comment on this.
I have asked Santander this very same question before I started accepting payments like this. They told me once the money is in my account there is nothing that would allow the money to be taken back, even if people have been victims of fraud the bank they have a hell of a time getting the money back.
Either it's a mistake or the person has not explained themselves properly or it's just a wind up. Even if you Google there is plenty of information telling you this simply cannot happen.