In the story of Cholkar, why did he give up sugar? Was that part of the original vow he made to Sai Baba?

No, giving up sugar was not part of Cholkar's original vow. According to Chapter 15, Cholkar's vow was that if he passed his examination and secured his job, he would go to Shirdi and distribute sugar candy (khadisakhar) in Baba's name. However, after his wish was granted, he faced a delay in fulfilling this vow due to his poor financial situation, described as the difficult 'threshold-ghat' of home expenses. As a personal penance for this delay, Cholkar decided to give up sugar entirely, even drinking his tea without it, until he could finally make the journey to Shirdi and fulfill his promise.