Ms project - Actual cost column

Hi all planners,

I am using ms project-2007 for my scheduling works. I have created a schedule with work resource,cost resource and did save baseline. Now i want to change the values in actual cost column of cost resource based on the site progress but when i tried to do so, i got a problem. When i increase the value of actual cost more than that of baseline cost , the duration of the particular activity is also changing.

I increased the actual cost because of more wastage of materials in the particular activity. I didn't spend more manpower or duration.

Is there a way to enter the actual cost without disturbing the duration of the activity?

What am i missing?

Thanks in advance.

Regards,
Ram