The counterfeit money that you are looking for is found in the middle of the building. It is somewhat close to the open window on the third floor of the Southern face of it.
There is a Watcher by the Northern window. Use Eagle Vision to mark them and wait for the nearby Brute to move away. Once the Brute does, enter through the window, eliminate the Watcher (and the Brute if they were closer than you thought). A Brute will patrol the floor and look out the Southern window.
Once inside, deal with the Brute if they get in your way. Head to the Southern side of the building.
Go into the Southern room and look to the East of the table by the Northern door. You will see the Counterfeit money you were sent after.
- Burn the counterfeit money
- Let the civilians into the bank