Who did Sai Baba tell to feed a black dog and why?

The provided texts do not mention Sai Baba, also referred to as Maharaj, instructing anyone to feed a black dog. There seems to be a misunderstanding in the premise of your question. However, Chapter 0 does describe a similar charitable act by a devotee named Kashiram. It was a different saint, Janakidas Baba, who advised Kashiram to feed sugar to ants. Kashiram took this advice so seriously that he always carried a bundle of powdered sugar for this purpose and even fought off robbers to protect it, highlighting how dear this practice was to him.