This would be classified as an incentive payment. When the payment is made directly to the salesperson by the vendor, then the employee would need to declare it as income on their tax return, but the real estate agent would not need to get involved. It would be seen as a separate arrangement and not part of their wage or salary.
However, if the payment has been made via the real estate agent, it is classified as relating to work having been done in ordinary hours; therefore you would put it through payroll and classify it as an incentive payment or bonus. It is liable for super and PAYG tax must be withheld before passing on final payment to the employee.