What choice does Assef give Amir?

Assef tells Amir he can have Sohrab. "What's the catch?" you may ask. Well, Amir has to fight Assef. (Assef still holds a grudge against Amir from the time Hassan pulled a slingshot on him. Assef said he'd get both Hassan and Amir – so now he wants to fight Amir.)

Assef said Amir could have Sohrab, but he would have to kill Assef in order to leave the house. He told his guards to wait outside the room and not enter no matter what they heard. If Amir killed him, he would be able to go free. Then Assef told Sohrab to stay as a "lesson."