Can you explain how a devotee's doubt was cleared by Sai Baba using grapes?

Certainly. This leela, described in Chapter 35, involves a devotee named Boss Dharamsee who was initially driven by curiosity rather than faith. When he visited the mosque, another devotee, Kaka, offered grapes with seeds to Baba. Baba began distributing them, and when he gave some to Dharamsee, Dharamsee was internally conflicted. He disliked seeded grapes and was also concerned they were unwashed. He thought to himself that if Baba were a true saint, He would know his dislike. Sensing his thoughts, Baba gave him even more grapes and told him to eat them. Upon obeying, Dharamsee was amazed to find the grapes were perfectly seedless. This direct experience of Baba's omniscience and power completely dissolved his doubts, and profound love for Sai was born in his heart.