In Mark Twain's narrative I sell a dog what is the narrator's main source of income?

Mark Twain at the time was a struggling writer. He and his friend needed three dollars for something urgent. As he tried to find a way, he saw a dog at a hotel. While playing with it, General Miles came and asked if he can buy it. He said it costs three dollars and sold it to the general. Later a man approached him and asked him if he had seen his dog. Realizing that he was the true owner, he looked for the general and asks to get the dog back. He returned the money and gave the dog to its owner who gave him three dollars as a reward.