Customer wanted to reprioritize activities and move them up the schedule.  This will result in some tasks being carried out in parallel. To avoid over-allocating my resources,  we have recommended to add one more resource and since this is a change from our original approved schedule and activities,  we will charge the customer the additional resource.  However,  they are telling us that it shouldn’t be the case,  as basically we have the same scope and the overall project schedule will actually now be ahead by 8 calendar days due to the parallel activities.  Since I’m quite new with PM,  I can’t explain why this is not the case.  What are the factors considered here and why shouldn’t the 8 days be considered as ‘saving’ and should instead be used to cover the cost of the additional resource as our customer is telling us?  My lead consultant is telling me about risks, opportunity costs, contingency, etc. but it’s not very clear to me. This is a fixed engagement btw,  not T&M.