I found it on Tumblr, and I THINK I solved it (with the help of looking at the notes LOL).
“You saw a shirt for $97. You didn’t have the cash, so you borrowed $50 from your mum and $50 from your dad = $100. You bought the shirt, and had $3 change. You gave your dad $1 and your mum $1 and kept the other $1 for yourself. Now you owe your mum $49 and your dad $49. 49+49 = 98 + your $1 = 99. Where is the missing $1?”
I did the math, and if the guy didn’t keep the dollar, he would have gave each of them $1.50. Which means, he owes them a total of $48.50.
So, 1.50×2=3. So he gave back a total of $3.
I think I solved it. But how the fuck did the person who wrote the riddle get 99? LOL.