Pro Forma Billing - Approved But Unreleased Invoices

We have had a few instances of unrecoverable billable items due to our current process on a couple of clients who require 'pro forma' invoices. 

We generate a draft and send it to them.  Once approved, it is finalized.  Invariably, the client delays approval, sometimes for many months, causing us to have to generate the next pro forma before the prior one is finalized.   Sometimes multiple months can span and then we have a real challenge getting the final invoices to match the pro formas. 

We attempted to generate the invoice, approve it, but not release it.  However, when we when generate the next invoice,the previous approved but unreleased invoice amount is added to a new invoice, combining both invoices into one invoice.