Can you tell me the story of the wooden plank that was given to Sai Baba and how he used it?

Certainly. As detailed in the Introduction, Chapter 0, Nanasaheb Dengle, seeing that the Masjid was dilapidated, brought a thick wooden plank for Maharaj to sleep on, intending for it to be placed on the ground. However, Maharaj used it in a unique and miraculous way. He hung the plank from the rafters of the Masjid using old rags and would sleep on it, placing four lit lamps at its corners. The rags, which seemed too weak, miraculously supported not only the plank's weight but also Maharaj's body. This spectacle drew large crowds, and because it became a distraction, Maharaj eventually broke the plank.