What was the specific vow Cholkar made, and what were the circumstances that led to his personal penance?

Cholkar's vow was born out of necessity. As Chapter 15 explains, he came from a 'poor household' and needed to pass an examination to make his salary permanent. He made a firm resolve that if he passed through Baba's grace, he would go to Shirdi and distribute sugar candy. While his wish was granted and he passed the exam, he was unable to immediately fulfill the vow due to financial hardship. This delay troubled him, so he undertook a personal penance: he completely gave up sugar, treating it as an 'inedible substance' and even drinking his tea without it until he could finally complete his pilgrimage to Shirdi and fulfill his promise to Baba.