Aww I am a bit disadvantaged here.... I don't know how much that's worth... I remember being terribly disadvantaged when I went to the states - all that foreign money and all.. I usually just handed over a pile of money and let the assistant give me the change :P
So if it were the equivalent to a UK pound, I'd probably keep it unless I Knew for sure he dropped it... If it were a penny, I'd keep it as we have that saying, 'find a penny, pick it up and all the day have lots of luck' so maybe we are programmed to take the money and run