Amir leaves money under a mattress for Wahid’s family- something he did for Hassan years ago. How is it different this time? What changes do people see in Amir this time?

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."