Find Item Number & Add To Another Workbooks Total

I have two workbooks, (Invoice.xls) and (Inventory.xls) both using sheet1. When I fill out an invoice, with lets say 10 items on sheet1 column B, I need a macro to find those items in workbook "Inventory" sheet1 column A. When the item is found, I need the qty I entered on the invoice workbook "Invoice" sheet1 column C (same row as the item number) to be subtracted from the current total for that item in workbook "Inventory" sheet1 column C. I would like to run the code from the workbook "Invoice" before I do a "Save As".
I have found other Vb code on this site that seems very close to what I need, but I just don't know enough to make the changes necessary.