What prompts Amir to write his first story?

After he changes the ending of a story he is telling Hassan, and Hassan says it the best one yet. Amir gets inspired and writes his first story about a poor man who became overcome with greed and ended up killing his wife. He shows the story to Baba, who does not really acknowledge it; and then to Rahim Khan, who writes Amir a note telling him what a great author he could become. Hassan later hears the story, and loves it. However, he introduces to Amir his first PLOT HOLE.