How did Rahim Khan "trick" Amir for Sohrab?

never read that Rahim Khan "tricks" Amir. Much later in the book he is in ill health and asks Amir to return to Afghanistan to retrieve Sohrab. He explains that Hassan and his wife were shot by the Taliban and Sohrab (their son) was sent to an orphanage. This was Amir's chance to atone for his guilt over Hassan's rape so many years ago. So, I'm unclear about the "trick". Anyhow I hope this helps a little.

No there was a trick. Rahim Khan told Amir to bring Sohrab and then keep him in an orphange owned by John and Betty Caldwell in Peshawar. However when Farid went to ask in the U.S. consulate he found out that they never existed.

Rahim Khan did so because he wanted Amir to atone for his and his father's sin by saving Sohrab.

Rahim Khan asks Amir to give Sohrab to a couple that owns an orphanage in Peshawar, their names are Thomas and Betty Caldwell. Later, when Amir is in the hospital (Chapter 23), Farid tells Amir that no one named Thomas and Betty Caldwell ever existed in Peshawar.

Rahim Khan does this so that Amir is forced to face his guilt about Hassan's rape years earlier. He is forced to rescue Sohrab and figure out what to do with him.