A lot depends on how you've structured your database.
Do you have a table for tracking inventory changes such as described in this thread? http://forum-en.filemaker.com/fm/board/message?board.id=FM-en-4&message.id=21633&query.id=166355#M21633
Have you set up your invoices with a portal of related line items where each line item records the item ID, quantity purchased, unit price etc.?
If so, you either create records in the inventory log that match your inventory line items in this "line item" table, or structure the inventory log table so that your line item records are actual inventory log records. You have to manage some details with this last option, but it can be made to work. You'd need a different layout for logging non-invoice inventory changes such as receiving shipments and shrinkage, but as long as the individual records are labeled as to type (Invoiced inventory changes will have an invoice number in an invoice field and the other transactions won't), it can be done and avoids having to copy data from one table to another.