I heard a story about a time when actual Udi wasn't available, but a devotee was still healed. Can you elaborate on what happened?

Certainly. This remarkable incident is detailed in Chapter 33. A friend of Narayanrao was stung by a scorpion and was suffering from severe pain. In their moment of need, they could not find any of Baba's Udi. Not losing hope, Narayanrao turned to a picture of Sai Baba and prayed compassionately. He then noticed the ash from burnt incense sticks that had collected at the bottom of the picture frame. In that desperate moment, he considered this ash to be Udi, took a pinch, and applied it to the sting while chanting Sai's name. The text wonderfully describes that as soon as the ash was applied, the pain subsided as if it were water flowing away, demonstrating the power of faith.