Where Did the Money Go?
The answer is that the question is deceptively asked. You have to be sure that you understand clearly what the situation is. The total amount of money spent is $250 for the box, plus $20 that the clerk kept, for a total of $270. The fans originally spent $300, but got back $10 each, so the total spent is $270 regardless of how you look at it, provided that you look at it clearly.
The closest solution to this question came from John Moser (firstname.lastname@example.org). He saw that the question itself was, in fact, quite easily misunderstood. His appetizing solution...
It's a matter of apples and oranges. What do you get when you add 3 apples and 4 oranges? ---- none of the above --- you get 7 fruits. When you compare the wrong things you get an impossible solution. The $270 is the $250 plus the $20 tip(?) therefore adding $20 again creates mass confusion but not a Math problem. Conclusion --- you are ten dollars short because 250 + 20 + 20 = 290 but has nothing to do with the original 300.