In "Kite Runner", why is it so important when Amir plants a fistful of money under a mattress on his trip back to Afghanistan?
Amir's action of planting a fistful of crumpled money under the mattress on his trip back to Afghanistan mirrors his action in planting his birthday watch and "a handful of Afghani bills" under Hassan's mattress twenty-six years earlier (Chapter 9). That first action was the culmination of Amir's cowardly betrayal of his loyal friend, the "sin" that would haunt him all his life. The deed, undertaken out of jealousy and guilt for his earlier act of treachery in not defending Hassan when he was being victimized by the degenerate bully Assef, determines the course of Amir's life into adulthood. It is testimony that Amir is indeed everything his father despises - "a man who can't stand up to anything" (Chapter 17).
When Amir plants the "fistful of crumpled money under a mattress" on his trip back to Afghanistan, his motives are different and entirely noble. The money is for Wahid and his family, who have treated him with hospitality despite their destitution (Chapter 19). Amir's action this time signifies his redemption - after leaving Wahid's house, he is committed to traveling the dangerous road to Kabul to rescue Hassan's son Sohrab. Amir has at long last found the courage to stand up for himself and those he loves, to act honorably and make up for the wrong he has done. Amir has found "a way to be good again" (Chapter 14).