Using Prepaid PO's Help!

I'm a small machine shop and I'm using QB2005pro. I have some work I send to outside vendors (this is new for me) and I some times send a deposit with a PO.I created an acct called PO Prepay and I enter the prepay amount on the PO as a negitive,as per QB's help suggestions.As my vendor delivers parts I recieve them against the PO and the inventory is updated, that part I seem to have down. I don't recieve all of the PO at once, it is delivered a few parts at a time, I am invoiced for the parts delivered,which usually means one invoice is enter against 3 or 4 deliveries,going through the deliveries one at a time and entering part of the invoice is a pain but it seems that's the way QB wants it.My problem is I still have a prepayment to log against the recieved goods/invoice, how do I do it. Generalized example of where I'm at. I sent a PO out for $50,000 of work, $4250 prepay deposit. I've recieved 7 deliveries of parts,about $20,000 of the PO, I recieved Invoice for $10,000 about 5 deliveries,Vendor would like a payment, currently pay bill shows $10,000 owed, but I have to subtract prepayment which means Pay Bill should show $5750 owed. Where do I go,what did I miss, QB help is not very helpful. Hope this is not too long and makes some sense. Thanks Steve