Laugh a little
Moishe goes into a restaurant and orders potato latke. When they arrive at his table, he does not like the look of them and changes his order to blintzes. Later, when he had finished, he gets up to leave.
"Wait a second," said the manager, "You haven't paid for your blintzes."
"What are you talking about?" Moishe replies. "Those blintzes were only an exchange. I gave you the potato latkes for them."
"Yes, but you didn't pay for them either."
"Why should I pay for the potato latke? I didn't eat them."