Generate Invoice and track down payment for work?

[Mark S]

I use GnuCash to track my small business.

I have a customer where I am requesting 50% of the estimated cost down 
up front.

Is there a way (and, if so, what is it) to generate an invoice for the 
50% down, record the income, but then be able to reduce the future 
invoices until this amount has been used up?

Thank you.
-Mark