The problem is simple but a bit confusing.
Actually there is no gain or profit from pastery owner as the shop keeper gives 100$ back to him.
So it becomes a simple problem.
My answer to this would be :
$30 paid out in cash to to the fraudster + cost of the ring to the seller
Logically speaking the seller would be selling the ring at Cost + Profit (which in this case is 70$)- So provided the seller bought the ring from a supplier for $50 before selling at $70 his loss amounts to 50+30=$80
The $100 taken from the neighbor in exchange for the voucher has been returned therefore no loss recorded to any of the parties here.
Please let me know the official answer if u have it :)
