Customer wants the cost to be fixed, not just the amount billed to the customer.

The scenario is that they want to have a fixed cost even if the duration is 20 or 22 working days.

Just like salary per month, if you work April 21 wd and May 22wd, the salary would remain the same for every month.

Customer mentioned that they used to test by creating project with fixed price but the cost would not be fixed