How did Sai Baba reveal his knowledge of Cholkar's secret vow, and what was the stated purpose of this revelation?

After Cholkar had finally traveled to Shirdi and fulfilled his vow by distributing sugar, he was visiting with a man named Jog. As they were about to leave, Sai Baba specifically instructed Jog to "Give him cups of tea, well-filled with sugar." This deeply moved Cholkar, as he had been secretly avoiding sugar as a personal penance during the delay in fulfilling his vow. Chapter 15 explains that the purpose of this miracle was to confirm Cholkar's faith. Baba, who did not even drink tea, made this comment so that "the stamp of devotion would be impressed" and Cholkar would receive a clear sign that his vow, the long delay, and his private sacrifice were all known to Baba.