Marys Box

John has $20. He puts it in a box.
Mary has $20. She also puts it in the box.
John says he will sell the box, with money inside, to Mary for $30
Mary agrees, giving John $30

John put $20 in the box and received $30 back from the sale, resulting in $10 profit

Mary put $20 in the box and paid an additional $30 to receive the box. This results in an outlay of $50 in order to receive the $40 from the box. Mary has $10 less than when she began transactions. Depending on the value of the box, it could be viewed as either a poor decision or wise decision.