Can you tell me about the miracle of the grapes described in the scriptures?

Certainly. In Chapter 35, a story is recounted about a devotee named Kaka who brought grapes with seeds to offer to Sai Baba. A skeptical visitor, Boss Dharamsee, was present and disliked this type of grape. When Baba offered them to him, Dharamsee was troubled and questioned in his mind if Baba was truly a saint, since he didn't seem to know his preference. In response to this thought, Baba gave him even more. Finally, when Baba insisted he eat them, Dharamsee put them in his mouth and was astonished to find they were completely seedless, a strange power that amazed him and turned his doubt into devotion.