Invoice number problem

just found a bug and not sure how to fix. This workbook is set up to advance the invoice +1 evert time you clear sheet and save changes. This is a master workbook, when I make a invoice from the master and save it to another name everything is ok. If I open the other name workbook it changes the invoice number to the current master number. Is it possible to have the saved other invoice number stay the same after you open it later. ' Thanks Z