Is there an instance in the Satcharitra where Baba appeared as a different Fakir to test a devotee's promise?

Yes, this is strongly implied in the story of Appasaheb in Chapter 33. After Appasaheb had made a verbal promise to give ten rupees, a stranger Fakir appeared before him. Because this Fakir was unknown, Appasaheb grew suspicious and did not offer the money, even though he had six rupees available at that moment. The author of the Satcharitra then poses a rhetorical question, asking why Baba would come in a Fakir's form if not as a pretext to collect the promised Dakshina and test Appasaheb's love and conviction. This shows that Baba could use various means, including appearing in disguise, to interact with his devotees and see if they would fulfill their commitments.