What is the story of Das Ganu and the holy rivers at Baba's feet as told in Chapter 22?

Chapter 22 recounts a beautiful incident involving Das Ganu. When Das Ganu asked for Sai Baba's permission to go on a pilgrimage to bathe in the holy waters of Prayag (the confluence of Ganga and Yamuna), Baba offered a profound alternative. As the chapter details, Baba instructed him, "Know this big toe of Mine to be Prayag; bathe right here." When a devoted Das Ganu placed his head upon Baba's feet in acceptance, the waters of both the Ganga and Yamuna miraculously manifested from Baba's big toe, satisfying Das Ganu's desire completely and demonstrating the Guru's immense power and grace.