I've heard that one should not go empty-handed to a Guru. How is this principle shown in the story of Kaka and the grapes?

The principle of not approaching a Guru, God, or priest empty-handed is explicitly mentioned and demonstrated in Chapter 35. The text states, 'It is not good to go empty-handed to the door of God, a priest, or a Guru.' Adhering to this custom, Kaka, on his way to Shirdi for Baba's Darshan, made a point to purchase an offering. He bought two seers of grapes that were available at the time, even though they were the variety with seeds. This act of bringing an offering, as detailed in Chapter 35, set the stage for the subsequent miracle that Baba performed for the skeptical Boss Dharamsee, turning the simple offering of grapes into a profound teaching moment and a demonstration of his divine power.