Check the basement window in the side of the general store, along the street and near the newspaper boy. Peek inside to see the suspicious activity and trigger this mission. Head out of the town a bit, wear your mask then head inside the store.
The store owner will open his basement and take you down. You can threaten him to tell you where he keeps his money or you can find it yourself using Eagle Eye. The money is stored on a hollowed part of his basement wall. If you no one sees your robbery in action and if you didn’t fire your guns at all, this robbery will go smoothly and cause no commotion (hence, no bounty) at all.