Far Cry 3 Unhappy Reunion follows the main story quest Lin Cong I Presume. It is time to give Buck the knife in exchange for Keith’s life.
Go to Buck’s hut.
Buck’s hut can be found at south side of Badtown. After going inside his hut a cut-scene starts.
Bring the knife to Buck.
After you make the exchange Buck will change his mind. In a event that follows it you will be faced to fight with Buck for your life. At this part it is crucial to enter the correct buttons shown in a short timely manner in order to land a clean kill. Soon after that you will bring Keith to others to the cave.