Separating Income From Paid and Unpaid invoices

[Crimson Corelio]

I'm using GnuCash to keep track of my consulting business, creating invoices
for all my clients.  I have an Income account names Income:Freelance that is
posted to by the invoices.  Now I post invoices whenever I send them out to
my clients.  So at that point they are entered into Income:Freelance.  As
they're paid, I enter the payment.  But for tax purposes I want to be able
to pull out just the paid invoices.  Is there an easy way to do something
like this?