What is the job of the person who stands to receive a substantial cut of the price of the letter?
On a moonlit, tropical night, the native workers are asleep in their outdoor barracks. A shot is heard; the door of a house opens and a man stumbles out of it, followed by a woman who calmly shoots him several more times, the last few while standing over his body. The woman is Leslie Crosbie, the wife of a British rubber plantation manager in Malaya; the man whom she shot is recognized by her manservant as Geoff Hammond, a well-regarded member of the European community. Leslie tells the servant to send for her husband Robert, who is working at one of the plantations. Her husband returns, having summoned his attorney and a British police inspector. Leslie tells them that Geoff Hammond "tried to make love to me" and that she killed him to save her honor.

Leslie is placed under arrest and put in jail in Singapore to await trial for murder; that she killed a man makes such a trial inevitable, but her eventual acquittal seems a foregone conclusion, as the white community accepts her story and believes she acted heroically.  Only her attorney, Howard Joyce, is rather suspicious. Howard's suspicions seem justified when his clerk, Ong Chi Seng, shows him a copy of a letter Leslie wrote to Hammond the day she killed him, telling him that her husband would be away that evening, and pleading with him to come—implicitly threatening him if he did not come.
Ong Chi Seng tells Howard that the original letter is in the possession of Hammond's widow, a Eurasian woman who lives in the Chinese quarter of town.  The letter is for sale, and Ong himself, whom Howard had believed to be impeccable, stands to receive a substantial cut of the price.  Howard then confronts Leslie with the damning evidence and she breaks down and confesses to having written it, though she stands by her claim of having killed Hammond in self-defence.  Yet Leslie cleverly manipulates the attorney into agreeing to buy back the letter, even though in doing so he will risk his own freedom and career.
clerk