Advice needed for billing

[Timothy Wight]

I am struggling to use the vendor billing for suppliers that require
advance payment.  The business process is as follows:

1)	We place an order for multiple items.  The vendor either replies with
a pro-forma invoice OR (more often) we are ordering using an online
system and know the order value immediately.
2)	Our overseas suppliers terms are usually 'payment with order' so We
transfer funds (make payment?) for the order value.
3)	At a later date the supplier ships those items he can fulfil from
stock and issues an invoice for the shipment.
4)	At further date/s the supplier ships back order items when available
and issues one or more invoices for these shipments.
5)	If the back order delay for an item is too great for our purpose we
may cancel the item, or adjust the quantity.

So the final collection of invoices are likely not to match the order
value.  Some items can be shipped/invoiced after subsequent orders
placed and paid.  This is further complicated by the use of currency
trading accounts and exchange rate changes throughout.

Can anybody offer any advice how to achieve this?