Final answer:
Amir hides the money and watch under Hassan's mattress to frame him for theft and alleviate his own guilt.
Step-by-step explanation:
In the novel The Kite Runner by Khaled Hosseini, Amir hides the money and watch under Hassan's mattress because he is trying to frame Hassan for theft.
Amir feels guilty about betraying Hassan earlier in the story, and this act is his way of getting back at him. By placing the items under Hassan's mattress, Amir hopes that his father will believe Hassan is the thief and that he will be punished, thus alleviating his own guilt.
Amir hides the money and watch under Hassan's mattress because he wants to frame Hassan for theft. The crisp banknotes and the glittering watch are tools for Amir to create a situation where Hassan is accused and forced to leave, thereby eliminating the source of Amir's guilt and jealousy.
The act demonstrates Amir's internal conflict and the extent to which he is willing to go to rid himself of the constant reminder of his own cowardice and betrayal. This pivotal moment in the narrative, much like the hidden treasure beneath the barber's feet in the parable from the Makhzan al-Asrar, becomes a metaphor for Amir's unacknowledged feelings and the wealth of guilty secrets he harbors.