On this page of the Assassin's Creed Valhalla Siege of Paris guide, you will find a detailed walkthrough of the Stealing From Thieves side quest.
Reward: unlocks A Package for Paris mission.
Starting the mission
This mission will start in the Medanta, in the northwestern part of the region.
This mission only involves picking up a crate located right next to the shore. There are several bodies lying around. All you have to do is approach the blue dot and pick up the package. This will complete this minor task.
Completing Stealing From Thieves unlocks A Package for Paris.