The incident was as follows: That the Prophet (ﷺ) gave him one Dinar so as to buy a sheep for him. `Urwa bought two sheep for him with the money. Then he sold one of the sheep for one Dinar, and brought one Dinar and a sheep to the Prophet (ﷺ). On that, the Prophet (ﷺ) invoked Allah to bless him in his deals. So `Urwa used to gain (from any deal) even if he bought dust.

Reference: Bukhari: 3642