Can you tell me the story Baba shared about the man who forgot his vow to give away his first salary?

Certainly. To illustrate his point about collecting past debts, Sai Baba told a story in Chapter 36. He spoke of a man who was once poor and, upon being offered a job for fifteen rupees, made a vow to offer his first month's salary to God. However, he forgot his promise. As his salary increased dramatically over time, eventually reaching seven hundred rupees, his forgetfulness grew stronger. Baba explained that the man's past good deeds brought him to Shirdi, and it was then that Baba, under the pretense of asking for Dakshina, requested the original fifteen rupees that were owed, thus helping the man finally fulfill his long-forgotten vow.