Split lines at invoicing?

I have a scenario that I haven't come across before and wanted to see if anyone had a solution short of re-entering sales lines or entering separate lines for each quantity ordered?

Here's the scenario:

The customer orders 10 of an item. Those 10 items are ship confirmed and sent to the customer. The agreement with the customer is that they will invoiced for those items as they are used. So, the customer reports that they have used 7 of the items this week. Is there a way to only invoice only 7 of those, leaving the other 3 open and available to invoice later?