Nasruddin would come to a town from outside with his donkey that carried hay or other good. *Trick of the Trade* is a tale of how a toll collector tries to find out how Nasruddin was getting rich with every passing day. He would check the donkey, the goods and Nasruddin. He suspecting Nasruddin was smuggling gold or diamonds. But the toll collector never found anything.
After the toll collector retired from service, Nasruddin told him that he was actually smuggling donkeys!