Did Sai Baba ever appear in disguise to collect offerings from his devotees? What does chapter 33 say about this?

The narrative in Chapter 33 strongly suggests this possibility through the story of Appasaheb. After Appasaheb mentally resolved to give ten Rupees, a "stranger" Fakir appeared before him. Appasaheb's suspicion caused him to hesitate and not give the offering. The text then poses a rhetorical question: "had there been no love for Appa, why would Baba come in Fakir form? If it weren't a pretext for Dakshina, what flavor would come to the story?" This line clearly implies that the visit from the Fakir was indeed an opportunity arranged by Baba for the devotee to fulfill his word, presenting it as a test of faith.