Let's do this step by step, we know that the guy steals $ 100
So it would be - $ 100 for the store.
The men buy $ 70 merchandise WITH THE MONEY THAT HE STOLE, but the money that he stole CAME BACK to the store
So up to this poin, : - $ 100 + $100 - $ 70 = - $ 70 for the store
After that , the store give $ 30 cash back,
So in the and : - $ 70 - $30 = - $ 100 lost for the store