Ladbros gave me £825 in error and I gave it back. Am I a mug?

Went into a shop in my local high road and placed £200 on Alsace Lorraine and took 11/8. When I returned and handed in the slip, the girl deferred to the manager who looked wearily at me and said give me 3 mins, I had put £25 e/w on Norwegian Dancer out the winnings so turned to watch the race. When I turned back I saw him with a huge stack of notes, and he proceeded to smack £1250 down on the counter!

Now I had split seconds to decide what to do, and had it been £50 overpayment, I might well have walked away...but I just found myself frozen, and fessed up. It turned out the 11/8 on the slip had been read as 11/2 by the computer, and I have to wonder if the error would ever have been spotted, and therefore no one would have got in trouble, but I wasn't to know that at the time, and I just couldn't do it.

I'm hoping karma will pay me out for this, but I doubt it.

Am I a mug? [;)]