Alright, let's walk through this scenario... Together! =D
So, this guy walks in this store like he owns the place. He just walks up, takes 100 bucks, and goes on his way.
However... This man forgot one thing. He forgot the new incredible toaster blazer that he wanted to buy!
So, he walks in. It costs $70, but that's nothing! He has the $100 bill from earlier to cover it.
So, he puts it on the counter - somehow he isn't recognised, and he gives them the $100 bill which mysteriously looks similar to the one they lost just 5 seconds ago, but no one notices.
Now, they give him $30 back in change - and now we're left with the question.
How much money did the store lose?
The store lost $100! Look at the other response, they explained it very well. =)