There are a number of side stories in Octopath Traveler 2 tied to Partitio and his The Scent of Commerce quests, and getting Mercantile Manuscript access in Sai is one of them. This is essentially access to a glossary and library offering insight about the world and the things in it. It’s also incredibly quick and easy to complete, if you have the time and 6,000 leaves.
First, make sure Partitio is in your party when you are in Sai. There is a small hill in town with a house on it. If Partitio is around, then he’ll recognize the insignia on a wagon on the hill. He’ll then realize it belongs to the legendary merchant Masoud, who wrote the Mercantile Manuscript.
However, Masoud won’t just give Partitio access. He wants a merchant’s greatest treasure. From there, head down the hill and approach the woman standing next to the well. This is Masoud’s daughter. Hire her as Partitio at night. It will cost you 1,000 leaves. Take her to her father.
You’ll then see more exposition. After the three talk, Partitio and Masoud’s daughter will walk away. She’ll then give you a hint as to what the answer to the “test” is. Partitio will also think about it and realize the proper response.
From there, head back up to Masoud at night. Have Partitio hire him for 5,000 leaves. This will complete the quest, and you’ll get the Mercantile Manuscript Archive Key in Octopath Traveler 2. This opens the door to the building behind Masoud in Sai.
Octopath Traveler 2 is available for the Switch, PS4, PS5, and PC. A demo is available.