So.....
The store loses $70 out the cash register and $70 worth of goods.
The money and the goods both belonged to the store....as the store already paid for the goods.
They have $100 in the register, and he takes $70, but gives them $30 back.
He takes $70 worth of goods.
They lose $140.
Even though he "bought" the items, he used THEIR cash to do it....so they got hit 2x.
Think of it this way.....if he took $70 worth of goods and just stole the $100, then we'd say the store lost $170. He gave them $30 back, so they lost $140.