I remember hearing a similar logic problem problem when I was younger (7-10). I can't remember it very well, but it does have the supposed "magically appearing dollar" thing.
Omega, I think that one was:
3 people (pirates, if you read the same book I did!) get a room at an Inn and each pay $10. The Inn Keeper realizes the room was only $25 and that he overcharged them. He tells the clerk to give $5 back to the people. Seeing that the people wouldn't be able split the change evenly, and given that they weren't expecting the money back any way, the clerk pockets $2 and gives the 3 people $3.
Each receives $1 back, and one says: sweet! we got the room for $27 (3 x $9). The clerk still has $2 in his pocket.
The room was $27. The clerk has $2. That's $29.
Where's the missing $1?