Why was Cholkar so astonished when Sai Baba mentioned giving him tea with sugar?

Cholkar's astonishment came from Baba's apparent omniscience. As Chapter 15 explains, after Cholkar passed his exam, he was too poor to immediately travel to Shirdi to fulfill his vow of distributing sugar candy. As a personal penance for this delay, he secretly gave up sugar entirely, even drinking his tea without it. When he finally arrived in Shirdi and was about to leave with his host, Jog, Baba specifically told Jog, "Give him cups of tea, well-filled with sugar." This revealed that Baba knew the secret of his vow and the private sacrifice he had made, causing tears of joy to fill Cholkar's eyes as his faith was profoundly confirmed.