"A plantation master in the north fields is known to be an unusually cruel and sadistic slave trader. Kill him."
Head to the North from the pigeon coop you accept the mission from. Head over to the Western side of the Restricted Area that appeared for this contract. Climb up on the building rooftop that sits there. get to the top of the roof and then use Eagle Vision looking East to find your target. Either shoot him with a pistol or a Berserk Dart to bring it to a swift end.