Head to the marked location and infiltrate the Imperial Spa. The easiest way to obtain the blackmail stash is from the rooftop. Head to the west until you reach the bathhouse. Don’t worry about the civilians here since they’ll just generally ignore you. Use your intel view often to keep track of the hostiles. Sneak inside and carefully perform stealth kills against the hostiles along your way. You’ll find the stairs leading to the rooftop in the second set of stairs to the west side of the building.
After reaching the rooftop, kill the guard in the small room to get some cash and access the weapon locker with the tac-vest. The rooftop has a sniping point and a sniper rifle on a nearby crate but you don’t have to do it for now. Move to the eastern side of the rooftop and kill the lone guard there. Enter the door in the corner, grab the cash from the shelf, and drop down to the dark room where two stacks of cash and the blackmail stash is located. You can now either leave the building or continue clearing the enemies inside to obtain more cash.