I borrow 50. From each of my parents = 100.00 to buy a shirt for 97.00 I get 3.00 back. I give one to each parent. Now owe each parent 49.00. Keep one for myself. But... 49+49=98+1=99. Where is the other dollar?

The "trick" here is to realize that you're adding when you should be subtracting. He owes his parents 2 x $49 = $98, representing the $97 shirt plus the $1 he kept. So instead of saying that $98+1 = $99, he should have said that $98-$1 = $97 (the value of the shirt).

This is a variation on the old riddle about the three guys who split the cost of a hotel room 3 ways - the room is $30 so the each pay $10. Then the manager realizes he's over-charged them by $5 so he sends the bellhop up to the room with the $5 to refund them. But the bellhop only gives them $3 and pockets the other $2. So that means it cost each of them paid $9, and the bellhop kept $2 - but $27 + $2 = $29, so where's the other dollar?