Why did Sai Baba send Madhavrao to London to collect a Dakshina of fifteen rupees?

The premise of your question contains a factual error. According to Chapter 18, Sai Baba did not send Madhavrao to London. Instead, Baba sent a messenger to Madhavrao (Shama) with a specific instruction. The message was for Shama to give fifteen rupees as Dakshina. The text states, "'Take fifteen rupees as Dakshina from Shama.'" When Madhavrao relayed this, he was surprised and responded by offering 'namaskar' instead of money. This incident, as narrated in Chapter 18, highlights the unique and often symbolic interactions Baba had with his close devotees in Shirdi, rather than any dealings in foreign cities like London.