If you want to pick up the Sniper Rifle and the suitcase at the same time, holster any weapons you may be carrying and pick up the suitcase. It should be in your left hand. Then, pick up the rifle and you will drop the suitcase on the ground. Take out you 9mm SD and the rifle should go into your left hand. Holster you 9mm SD, then pick up the suitcase. You now have the suitcase in your left hand, and have shifted the rifle to your right. You can even use the rifle with one hand.
Get the briefcase but do not kill the general. Instead, run to your boat and drop the briefcase directly next to it. Go back, kill the general, then get the rifle. Next, go up to your boat the mission will be completed. Try different positions for the briefcase if you do not get it the first time.
