As the "Inventory Log" originator, let me see if I can provide some further clarification. The first key concept, is that you currently, (If I understand your post correctly), have two tables, where you should have one table and another table appears to be missing entirely.
Here's the basic relationship design most invoicing system use and which we can also use for our Inventory Log system for tracking inventory:
Invoices::InvoiceID = LineItems::InvoiceID
Products::ProductID = LineItems::ProductID
A simple demo file you can download and examine is Comment's oft recommended Demo file: http://fmforums.com/forum/showpost.php?post/309136/
You use a portal to LineItems on the Inventory layout to log each item purchased. Price and description data is pulled from the Products table.
To get an Inventory Log system to work with the above setup, you use the LineItems table as the data source table for you Inventory Log. Each time you enter an item in the LineItems portal, your "quantity" field is the "ItemsOut" or "Credit" field of your Inventory Log. Then you can set up a separate layout to your LineItems table where you will see the purchased items automatically appear and where you can log all other inventory changes such as new shipments received or inventory shrinkage.
I think I had all the elements there, just named differently. Using Inventory Items as Products...and that "Inventory Log" I actually envisioned this is working exactly the way that this Line Items table works. Since this database is for an artist who is selling one of a kind pieces, I don't really need to keep track of new shipments or shrinkage, but the portals I now have set up in Inventory Items and Invoices work perfectly.
Thank you Phil!