Can you explain the specific details of the miracle involving Boss Dharamsee and the grapes?

Certainly. The miracle is detailed in Chapter 35. When grapes were offered to Sai Baba, he began distributing them. A visitor, Boss Dharamsee, received some but was inwardly displeased because they had seeds, a variety he disliked, and were unwashed. He mentally questioned how Baba could be a saint and not know his preference. Aware of this, Baba gave him more and instructed, "Throw them in and eat." When Dharamsee obeyed and put the grapes in his mouth, he was greatly surprised to find they were all seedless. Chapter 35 notes his amazement, realizing that what Sai gives becomes beneficial, and this experience transformed his doubt into devotion.