Let's put it this way, suppose you're walking down the street and you notice a bright red Ferrari 458 convertible. The roof is down, it's unlocked and the keys are in. So would you take it, since someone's left it lying there?
OF COURSE NOT. Not unless you are a downright criminal and you've just been released after your 19th conviction for theft. Most people would not take the car, not only because they would fear being caught and prosecuted, but because they know it is obviously not their's and taking it would be wrong.
Like the £300 is obviously not theirs and taking it would be wrong.