Amir leaves money under a mattress for Wahid’s family - something he had done for Hassan years ago. How is it different this time? What changes do readers see in Amir based on this action?

I think this is just an extension of Amir's guilt over Hassan. I think Amir just wants to show his appreciation for Wahid as well. When Amir stays at his house, Wahid is kind to him and does not judge him for being American. He calls Amir "a true Afghan."