Can you recount the story Sai Baba told about the man who owed fifteen rupees?

Certainly. In Chapter 36, to illustrate his point about collecting on spiritual debts, Sai Baba told Shama about a specific devotee. Baba explained that this man was initially poor and when he was first offered a job for fifteen rupees, he made a vow to give his first month's salary to God. However, he later forgot this promise. As his salary increased from fifteen to thirty, then sixty, one hundred, and eventually seven hundred rupees, his forgetfulness grew stronger. Baba stated, "He arrived here by the power of his past deeds. Then, I asked for my fifteen rupees under the pretext of Dakshina," thereby helping the man fulfill his forgotten vow and clearing his debt.