Let X = the price each guy paid (which is 10 of course)
We'll see if it works out.

3X = 30

subtract the 5 from the price:   3 X - 5 = 30 - 5

3X - 5 = 25

subtract the skimmed 2 bucks from the refund: (3X - 3) - 2 = 25

transfer the skimmed ill-gotten booty to the other side of the equation: 3X - 3 = 25 + 2

3X - 3 = 27

Divide all parts by 3: 3X/3 - 3/3 = 27/3

X - 1 = 9

X = 9 + 1 = 10

It all works out. Georgiansilver was right. You need to subtract the 2 bucks pilfered, not add it.
                        

The other one is a philosophical conundrum, a Zen Koan, not a scientific question. Of course the falling tree makes a sound.
The more interesting question is: Does the effect of the listener aprehending a sound and interpreting it as a falling tree change the result of the experiment?