Why did Sai Baba accept 15 rupees from one man from Goa but reject 35 rupees from his friend?

The incident involving the two gentlemen from Goa is detailed in Chapter 36. Sai Baba asked one of the men for exactly fifteen rupees, which he accepted. When the other man offered thirty-five rupees of his own volition, Baba refused it. When Shama questioned this, Baba began to explain that he doesn't take anything for himself. He stated, "Mother Masjid demands what is owed, and the giver becomes free from debt." This implies that the acceptance of the fifteen rupees was not a simple request for a donation but the settlement of a past, unseen karmic debt that the first man owed, while the second man had no such debt to clear at that moment.