Go inside the mansion and into the ballroom. Find the tall agent that is by himself next to the piano. Wait until the Ambassador (as shown in the photo) enters the room to talk to the agent about the briefcase inside the safe. After they are done talking, they will exit the ballroom and the Ambassador will escort the agent to the room with the safe. Follow them, but do not get too close. They will enter the room with the safe and will talk to each other before the Ambassador opens it. Go inside the room next to the room that they are in without looking suspicious. Look inside the keyhole in the room that you are in, and you can see the agent pointing a .54 pistol at the Ambassador to kill him. Open the door and wait until the Ambassador opens the safe. This is your chance to take out the agent silently. Take the briefcase inside the safe and exit the room. The Ambassador will be directly in front of the table, very scared.