Did Sai Baba receive a secret mantra from his Guru as soon as he started serving him?

No, he did not. Chapter 19 explicitly states that despite his intense service, his Guru did not immediately grant him a secret instruction or 'ear-mantra'. Baba recalls, "I wore myself out serving him, yet he wouldn't give me a 'ear-mantra'." He had a strong desire for one, but his Guru first tested him by shaving his head and asking for 'two paise'. It was only after Baba provided these metaphorical coins of Faith (Nishtha) and Patience (Saburi) that his Guru was pleased. This narrative demonstrates that the Guru's grace and secret teachings were earned through the cultivation of essential virtues, not given away lightly.