Justin, a real estate salesperson with City Brokerage, received a referral fee after referring a client to Mark, another real estate salesperson with City Brokerage. Who ultimately pays Justin his referral fee...?

Justin's sponsoring broker

Explanation:

From the question, we are informed that Justin, a real estate salesperson with City Brokerage, received a referral fee when he refered a client to Mark, who is another real estate salesperson with City Brokerage.

The person to pay Justin his referral fee will be Justin's sponsoring broker. It should be noted that when another agents gets a referral from an agent, the sponsoring broker is the one who gives the referral fee to the referring agent.