I heard a story about Sai Baba and some grapes. Can you tell me what happened to the devotee who was skeptical?

Certainly. The story involves a devotee named Boss Dharamsee who visited Shirdi with a skeptical mind, described in Chapter 35 as having a 'clinical scrutiny.' When Kaka offered grapes with seeds to Baba, Baba began distributing them. Dharamsee disliked this variety and was hesitant to eat them unwashed. He thought that if Baba were a true saint, he would know his dislike. As described in Chapter 35, Baba, seemingly aware of his thoughts, gave him even more. When Dharamsee finally ate the grapes upon Baba's command, he was astonished to find they were seedless. This miracle completely transformed him, causing his ego to drop away and a deep love for Sai to be born in his heart.