Can you recount the story of the devotee who made a vow to Lord Datta and how Sai Baba was involved?

Certainly. As detailed in Chapter 36, a man seeking a job to support his family went to Goa and made a solemn vow to Lord Datta. He promised that if he were granted a job, he would offer his entire first month's salary. His prayer was answered, and he began earning a salary of fifteen rupees, which later increased with promotions. However, the memory of his vow was completely forgotten over time. When he later came to Sai Baba, Baba used the pretext of asking for Dakshina to remind him of this old, unfulfilled promise. It was explained that this was not Baba begging, but rather him facilitating the repayment of a forgotten debt.