In The Kite Runner, why does Amir place money under a mattress when he returns to Afghanistan as an adult?
In The Kite Runner, Amir sees his friend Hassan get raped. Unable to deal with his guilt at doing nothing to help Hassan, Amir frames him for theft, causing his father to take him away out of shame. Years later, when he returns to Afghanistan to find Hassan's son, he makes a symbolic gesture for this misdeed:
It occurred to me that, in a different world, those boys wouldn’t have been too hungry to chase after the car.
Earlier that morning, when I was certain no one was looking, I did something I had done twenty-six years earlier: I planted a fistful of crumpled money under a mattress.
(Hosseini, The Kite Runner, Google Books)
Amir cannot go back in time to save Hassan, or undo his betrayal, but by giving some money to a poor family, in a method that recalls his misdeed, he allows himself some small measure of forgiveness.
This moment is Amir's attempt at redemption. Because Hassan has died, Amir will never be able to beg forgiveness of his childhood friend. By placing the money under Farid's mattress he is symbolically acknowledging his guilt for the exile of Hassan and Ali from Baba's house and, at the same time, he is using his position of wealth to fortify, rather than destroy, someone's life.