Do we have to pay repair man for ‘callback’ jobs he should have fixed the first time?

Q. In the heating and air business, our service technicians sometimes do not fix the problem the first time when at a customer's home. When we get a "callback," the same technician has to go back out and fix the problem he didn't fix the first time. Can we not pay the technician for having to go on callbacks for something they should have fixed the first time? If we have to pay them, do we have to pay the normal rate or can we pay a reduced amount? Thanks. – John, North Carolina