Yesterday at work, I was going out for lunch and the receptionist stopped me and asked if I would get her a sandwich. She gave me what I thought was a five pound note. I put it in my pocket to keep it separate from my wallet.

I got to the shop and reached into my pocket and pulled out a tenner. I immediately thought, 'OK, I must be mistaken, she must have given me ten pounds'. So I went back and gave her change for a tenner.

Just as I left work, I put my hand in my coat pocket and pulled out a fiver -the one she had actually given me after all.

So today I asked another colleague whether it would be reasonable to ask her for the fiver back and she said 'yes, she's very honest, it'll be fine'. So I did. She, admittedly very politely, insisted that she did give me a ten pound note.

I've gone over and over it in my head and am literally as certain as its possible to be about this, and I know she thinks she's right, and isn't trying to rob me but she is quite simply mistaken.

What the hell do I do? Leave it as it is - a bit awkward, or pursue it and risk turning it into a full blown argument?