Dear Trevor:

I have the same problem as Gary. I need to use only fixed cost.  I just change the default setting in Tools, Options, Calculation ("actual costs are always calculated by microsoft project"). I can edit the Actual Cost and I type $45 but the remaining cost not change, "remaining" $100 and Total Cost field increase to $145.  Please, let know what is missing.

Gary,
I would say that your approach is correct, that cost is independent of, or at least only weakly connected to, duration. Duration, Cost and Work for any task are each/all independent and all have Actual, Remaining, Total and %. Suppose there is a single 10 day task, with no resources assigned and therefore no work. It can still have a Fixed Cost of say $100 and this is the only source of cost for this task. When I track progress, I start with nailing down the actual start and actual duration because for sure any or whatever actual work or actual cost that has occurred has occurred on days of actual duration.
If the task is finished they must have occurred between the actual start and the actual finish. If the task has started but is not yet finished then any or whatever actual work or actual cost that has occurred has occurred on days of actual duration between the actual start and the status date.
So for this task, let’s say that the status date is at the end of what should be (ie is planned to be) the 6th day.
I always have the tracking table visible because it has the necessary fields. If we assume that the task actually started on the planned start date and has been continuous up to the status date, then there are 6 days of actual duration. The second last column/field in the tracking table is the actual work and the last column/field is the actual cost.
By default MSP calculates the actual cost, in this case $60, and you can’t change it. You select the cell but can’t edit it. But you can change the default setting in Tools, Options, Calculation ("actual costs are always calculated by microsoft project"). Now you can edit it. Just type in, say, $45 over the $60 calculated by MSP.
The total cost does not change. It is still $100. But the actual cost is $45 so the remaining cost is $55 (see the cost table). So the % Complete, always about duration only, is 60% (6 days actual/10 days total). "% cost", which doesn’t exist as a built-in field, but exists anyway, is 45% ($45/$100).
True, as you say, if you switch to the cost table, you find that you can select, but not edit, the remaining cost. But you can force remaining cost to change by changing the fixed cost or by adding resources which have costs. There is no calculaton option to change to allow it. I have always accepted this as MSP functioning as it should, as intended and as designed. It works and does not feel like a kludge work-around.