John Zahl writes in "A Gift (Certificate) That Never Expires”:

A few years ago, a friend and the owner of a local high-end department store gave me a very generous gift certificate. When I went to use the gift certificate he met me at the store, and walked with me as I selected a sports coat, a dress shirt, and a pair of shoes. I made sure to look at each of the price tags (on the sly) so I could overshoot the gift certificate enough and put some cash back into the store's register, thereby showing my gratitude for his generosity.

When I got to the register, I put my wallet on the counter and got out my credit card, but he placed the gift certificate in front of me and said, "Well, it looks like you've only spent a little more than half of your credit with us." I was shocked. In that moment I realized he had only been charging me half of the ticket price, which meant that I was still in his debt.

In a few weeks I returned to the store with my wife determined to show my appreciation by overspending the gift card. So this time we approached the counter as a unified front, and with a huge armload of clothing and accessories. I handed our friend the gift certificate, and got my wallet out. He took the gift certificate in hand and started entering the purchases into the register.

Finally, when the bags were full, he turned to us and said, "You're not going to believe this, but I've rung everything up, and the total comes to exactly zero." We started protesting: "That can't be right. The total should be well above what was left of our store credit."

Then he said, "I don't think you understand how this gift certificate works. No matter what you throw at it, the total will always read zero." We finally understood his arrangement. In our attempts to buy our way out of the debt, we had completely missed the value of the gift, which this generous man took such pleasure in bestowing upon us.