The cash option does NOT work, the ammo trick, writtem above, not work either. When I put military ammo in the clip, shoot some then reload, the ammo in my storage does NOOOOOOT change, only the clip reloads, so the overall amount of special ammo (cash) stays the same.
Edit: The cash/military ammo trick does not work in peaceful areas, like stations with weapon dealers and shooting range. The cash/ammo trick works only in areas where enemies can be found.