Microsoft Project automatically updates actual costs as a task progresses based on the accrual method you set. First, you must turn off the automatic updating of actual costs and then, if you want to enter your own actual cost for a resource assignment, you can do so after the remaining work is zero.
To update actual costs for a resource assignment
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Sometimes resource assignment costs include a per-use cost for items such as equipment costs, setup charges, delivery, or rental fees, in addition to a standard rate. You can change the fixed amount charged to a resource each time it is assigned to a task by entering a new cost in the Cost/Use field for the resource whose per-use cost you want to change.