For example, in case of visiting professor to a college would have following services provided. He is not a regular employee, hence no salary structure defined.

Weekend Class with charge of 2000. Invoiced to college here.

Conducting Project review with the charge of 1000 per student. Invoiced to student here

Payment duration is per month. Now in a month he conducted 5 weekend classes and 5 project reviews. Now the college needs to pay 10000 for weekend classes and 5000 for reviews, totaling to 15000. How can we solve this problem?