I would defer to the billing terms of the particular Clinical Trial. In many cases, a fixed fee is assigned to points in a study such as number of visits or a particular deliverable, so it isn't the actual costs that are itemized on the invoices. Rather, the revenue received for the deliverable or for the study visits is used to cover the actual costs and may result in a balance if the costs come in under budget..
However, if you have a trial that is set up as a cost-reimbursable award and not a fixed price or fixed fee payment structure, then I would assume only the true actual costs could be billed to the Sponsor. It will all depend on the terms of payment set up in the award.