Create a Status field that updates from Invoice

Another question,

So I have my inventory database working with an ability to create invoices for checking out equipment. However I need to add 2 things to the database. I need a field in the invoice table that has a status of either active or closed that auto updates in relation to the line items in the portal, when the quantity in equals the quantity out for the line items. Another part I need to add is a field (i'm guessing) on the inventory page that changes when the item has been added to an invoice, so that I know if an item is checked out, and then reverts back when the item is checked in, and make it dependent on the quantity since some items share a generic barcode and not a unique one. Here is a link to the database file for those who want to help. http://we.tl/PUGxsyBNmJ