I suggest opening the FMP 11 invoices starter solution to see a working example of how to make this work.
You need to set up a field in your invoices line item table that auto-enters (copies) the price from your inventory table. Since you have a copy of the price, future price changes will not affect past invoices. The typical table/relationship (data model) for this is:
Invoices----<LineItems>-----Inventory (called products in the starter solution)
You use a portal to LineItems to list the items purchased on a given invoice and a price field in LineItems looks up the price from Inventory.
This helped - thank you - I realized that I was using a field I created on the inventory page to override the list price - instead I used a look-up field in the line-items table on the invoice side. I then created a portal on the invoice page to find all invoices that "sold" the item, so I can quickly see the sales history for the item.