After entering cost rates and fixed costs for resources, you are unable to enter the actual costs.
By default, Microsoft Office Project automatically calculates actual costs according to the actual work accumulated or material consumed on tasks. If you want to edit actual cost information, you need to turn off the automatic calculation of actual costs.
Note: If the assignment on a task is completed (remaining work is zero), actual costs for the assignment can be entered in any view that has a cost table applied.
On the Tools menu, click Options, and then click the Calculation tab.
Clear the Actual costs are always calculated by Microsoft Office Project check box.