What punishment did Sai Baba give Bapusaheb Dev for writing a doubtful and accusatory letter?

The source material does not indicate that Sai Baba punished Bapusaheb Dev in any way. In fact, Chapter 40 describes Baba's reaction as one of surprise, not anger. When Jog read Dev's letter, Baba simply clarified the situation, explaining that he had indeed attended the feast as a Sanyasi. He instructed Jog to tell Dev, 'You did not recognize Me! Why then did you call Me?' The resolution was not a punishment, but an enlightenment for Dev, who, upon hearing the truth, felt deep remorse and recognized the flaw was in his own perception, not in Baba's actions.