Properly recording mixed/variable Invoice payment

[Jason Cooper]

All,

I've been using GnuCash to manage my company's books for almost a year
now.  Recently, my primary customer (whom I have little control over)
started mixing several reimbursements into one check.  So the history of
events goes like this:

15th: Send Invoice for services rendered this month
25th: Receive check for Invoice amount *plus*:
	- Any reimbursable expenses approved before the 9th
	- Any per-diem accrued to date

The kicker is that I have no idea which expenses are going to be
reimbursed this pay period, nor do I know how much per-diem they
calculated I should get (based on travel requests).  So I can't reflect
any of that when I cut the Invoice.

How on Earth do I track this correctly given the lack of information at
Invoice time?

thx,

Jason.