You need more tables to do this.
Typical invoicing systems have this structure:
One client can have many invoices; one invoice can have many line items and many line items can refer to the same type of item listed in inventory. The invoice template that comes with filemaker 11 can give you a fairly decent example of this.
You can place a Portal to LineItems on your Invoices layout for listing each individual item a given client purchases. In Inventory, a calculation field such as: Sum ( LineItems::Qty ) will compute the total items sold for all invoices.
You can also add additional fields to LineItems and use this table to track all inventory changes such as receiving new shipments or logging inventory shrinkage in a ledger format.