There's a story about Sai taking Dakshina from one person but not another. Can you explain that interaction?

Yes, this event is recounted in Chapter 36 of the Shri Sai Satcharitra. When two gentlemen from Goa came for darshan, both prostrated at Sai's feet. Sai then specifically asked one of them for fifteen rupees in Dakshina, which the man gave happily. The second gentleman, unprompted, then offered thirty-five rupees. However, Sai Baba immediately rejected this offering. The text highlights that this action caused great surprise among the onlookers, prompting Madhavrao (Shama) to ask Sai for an explanation regarding this seemingly partial behavior.