I heard Cholkar gave up sugar. Why did he do that, and how did Baba show him that he knew about this secret sacrifice?

Yes, after Cholkar's wish to pass his exam was granted, he was delayed in fulfilling his vow to distribute sugar candy in Shirdi. As a form of penance for this delay, Chapter 15 states that Cholkar avoided sugar as an inedible substance and even drank his tea without it. When he finally arrived in Shirdi and was a guest of Mr. Jog, Sai Baba demonstrated his omniscience. Baba, who did not drink tea himself, specifically instructed Jog to give Cholkar "cups of tea, well-filled with sugar." This meaningful instruction amazed Cholkar, bringing tears to his eyes, as it confirmed that Baba knew about his secret renunciation.