Down Payment or Prepayment Process

Hi all. I seem to be striking out on my forum posts lately, but thought I'd try one more:
In researching ways to improve our processing, the issue of what we call "partial billing" or prepayments has come up. I was hoping to get insight into how others handle these situations. Here's what I'm talking about:
We have a need to cut a PO to a vendor and the vendor is demanding a partial or down payment of 80% up front. Then, once the installation is done we will get another invoice for the remaining 20%.
Right now, we handle this by just putting in the PO as a service-type line so we can then get their 80% invoice in and match it to the PO.
But, looking into the prepayment option in Lawson, I was hoping we could maybe use that. The only problem is that when we use prepayment via AP206, it doesn't reduce the open-to-match amount any.
So, how do others handle these downpayment scenarios?