One dollar disappeared.... O.o

[Darkfader]

So here is the situation.

You find shirt which costs 97$. You loan 50$ from your mom and 50$ from you dad. You buy shirt and get 3$ back. You give 1$ to dad and 1$ to mom 1$ left for yourself.

Now you own 49$ to your dad and 49$ to your mom, 49$ + 49$ = 98$
And you have that one dollar so, 98$ + 1$ = 99$

Now that one dollar disappeared to somewhere. ;]

[Cascade]

You dropped it after you bought the shirt.

[thequickone]

Based on the repayment example you provided, the $1 you have should not be included as "owed", but rather treated as personal profit and subtracted or given back as part of the repayment. Doing so would look like this:
$49 (owed to mom) + $49 (owed to dad) - $1 (should have been paid back to mom or dad earlier, you cheap-skate) = $97 (the original cost of the shirt)
$97 + $3 change received at time of purchase = $100
No money lost, just poor accounting practices. ;)
Oh, and the shirts you are buying are waaaaaaay too expensive! :D