Barefoot Giant

(from ebookjapan):

This is an action adventure drama about a boy, Kuro Hata, who struggles to become a big merchant in Africa, where civil war is raging. Kuro comes to Africa on a small yacht with the ambition to make a lot of money, and Gonta Hatake, who falls in love with his manliness, follows him.

Later, Kuro and his friends visit the office of Murata, a trading company employee they know, and discover a dismembered corpse and...!