What is the principal conflict in the story, and how is it resolved?
Note the excellent summary of this story's themes at the link provided below.
A conflict always brings choices. What choice must Daru make? To follow the French colonial system that insists he turn this Arab prisoner (who has killed his cousin) over to the French authorities. Balducci, as a representative of that system, advises Daru to follow the colonial land. Daru refuses, arguing he doesn't want to be part of a system where "every bit" disgusts him.
Part of Daru's refusal to follow the law involves not restraining the Arab (in the hopes the man will escape and "solve" the situation for Daru) and then when the prisoner does not escape, allowing him to choose the road to prison or the road to nomadic tribes.
The resolution is that the Arab prisoner chooses the road toward the French police headquarters. How is his choice a resolution with consequences?
For the prisoner, he faces certain imprisonment if not death. It is unclear whether Daru really even gave him a choice (as he thought he did) by offering the road toward nomadic tribes, supposedly welcoming. Would this man be guaranteed safety, especially if he had killed someone? By the choice being murky, it makes the conflict that much more painful. In other words, no one really can win. (To learn more about Camus' Absurdist philosophy, consult the link below.)
For Daru, he faces certain danger if not death. Written on his blackboard is the threat, "You handed over our brother. You will pay for this." The prisoner's friends or family leave this note, assuming that Daru is guilty of sending the prisoner to his end. Daru, while thinking he was doing the right thing, is held accountable for it being the wrong thing, and there seems to be no hope for him. The denouement is him, an isolated figure against the landscape, "alone."