You want to buy a phone( the item is not important, it can be anything) that costs 97 $. You borrow 50$ from your mother and 50 $ from your father. After you buy the phone you are left with 3 $. You give 1 $ to your mother and 1 $ to your father. You now owe them 49 $ each. But you still got 1 $ .
Maybe its quite easy and I just don't see it.