Minimum wage and Disabilities question

Disabled individual ( as an individual contractor) agreed on say 20 hours of work for a job. During the course of work, employer wanted as part of the delivery to put information on an excel sheet. Disabled candidate in reality had no idea how to format documentation on excel which is commonly assumed and delivered the work on excel sheet but it took 60 hours to complete. Disabled candidate is asking to be paid on 60 hours of labor and not 20 hours agreed on before. What should Employer do. The Disabled candidate was provided commitment of 10% above minimum wage.