Why is putting the money under Farid's mattress so important in Amir's journey in The Kite Runner?

I'm sure you will remember that one of Amir's betrayals
involved the planting of money under Hassan's mattress in an earlier chapter of
The Kite Runner. Amir celebrated his birthday by contriving to
blame Hassan of the supposed theft of this money. The shame it brought Hassan and Ali
when the money was found prompted Ali to pack up and move away from Baba's house.
Placing the money under Farid's mattress was a symbolic way of cleansing Amir's sins
from the previous episode, and he knew the money would help Farid's war-torn and poverty
stricken family. Doing so anonymously and knowing it would be found after he left was
also a way of forcing the family to keep the money without feeling obliged to Amir or
sacrificing any sense of pride in accepting charity from their
guest.