Can you tell me about the time Sai Baba turned grapes with seeds into seedless ones for a skeptical visitor?

Certainly. This incident involved a man named Boss Dharamsee, who visited Shirdi out of curiosity about miracles. As recounted in Chapter 35, when a devotee offered grapes to Sai Baba, Baba began distributing them. Dharamsee, however, disliked that variety because they had seeds and were unwashed. He internally questioned Baba's sainthood, thinking that a true saint would know his preference. In response to this unspoken thought, Baba gave him even more grapes and instructed him to eat them. When Dharamsee complied, he was astonished to find they were all seedless. This personal miracle completely dissolved his skepticism, causing his ego to drop away and be replaced by love and devotion for Sai Baba.