Can you describe an instance where a devotee used extreme fasting to get instruction from Sai Baba?

Yes, there is a story about an elderly woman named Radhabai Deshmukh who was determined to receive a secret mantra directly from Sai Baba. As described in Chapter 18, she made a firm resolve and undertook a "dharna," a fasting protest, giving up all food and even water. She continued this for three days and nights, becoming exhausted and despondent, and was prepared to fast unto death until Baba gave her the instruction she so deeply desired. Her faith in receiving the instruction was immense.