the overall balance sheet would look like this:
step mom dad you store
0 50 50 0 0 start
1 0 0 100 0 you borrow 100
2 0 0 3 97 you buy shirt
3 1 1 1 97 you give them back 1 dollar each
the accounting is correct.
the store has 97 dollars and you and mom and dad have 3 dollars.
you still owe mom and dad 49 dollars each.
once you pay them, the accounting will be as follows:
4 50 50 -97 97 you pay them back 49 each
which is the balance you
owe them.
they get their money back and you are left with a deficit of 97 dollars which is the price of the shirt.
the 1 dollar that you have left is not related to what you owe them.
3 dollars is the difference between what you paid for the shirt and what you had in your pocket at the beginning which was the 100 dollars they lent you.
you still owed them 100 dollars and paid back 2 and you still owe them 98 dollars.
your 1 dollar left has nothing to do with what you owe them.
it's mixing apples and oranges.