Cost Issue when importing contractor schedule into existing project in P6v7.

Typically, when reviewing contractor schedule, I import their newly submitted file into the existing file that we maintain for that project, but when doing so, if the contractor has made any changes to the expenses during the update, they seem to add on top of the current expenses for example, Total Budgeted Cost originally is $15,000 , but the contractor moves expenses around between activities (eg. moves $500 to another activity). When schedule is imported, the new Total Budgeted Cost shows $15,500, even though it shows $15,000 (The correct amount) if you import and open the file as a seperate project. What can I do to prevent this from happening when I import the schedule into the existing schedule file?