Can you explain the events leading up to Boss Dharamsee's miraculous experience with the grapes in Shirdi?

Certainly. The story from Chapter 35 begins with Kaka deciding to visit Baba. Remembering that one should not go empty-handed to a Guru, he bought two seers of grapes, which happened to be a variety with seeds. He, along with Boss Dharamsee, went for Baba's darshan at the mosque. Dharamsee, who was there out of curiosity, was skeptical. When Kaka offered the grapes to Baba, Baba began distributing them to the devotees present. He gave some to Dharamsee, who disliked that particular variety and was hesitant to eat them unwashed. He ate the first few but held the next batch in his hand, troubled by his dislike. It was at this point, with Dharamsee's doubt at its peak, that Baba commanded him to 'Eat them up,' setting the stage for the miracle where the grapes turned out to be seedless.