The Briefcase is a key item that appears in Paper Mario: The Thousand-Year Door.
The Briefcase belongs to the Ratooey Businessman and was stolen during the events of Chaper 6. On the Excess Express, Doopliss was the one whole stole the Briefcase as he, Beldam, and Marilyn needed the Nitro Honey Syrup from it to destroy the train. However, Mario is able to get the Briefcase back and returns it to the Businessman.
Later, the Businessman needs help in opening his Briefcase as one of the troubles. The Businessman can't remember the combination and tells Mario that in order to find the combination, he must count the number of chairs in certain homes in a certain order. The combination is 2625 and, after doing so, the Businessman is able to sale Hot Sauce to Mario for 10 coins.