If it were me I wouldn't go back and modify picking orders that have already been validated, though I've never heard of a purchase agreement where the vendor can retroactively dictate changes to the terms of the contract after product is delivered. Even so, that still doesn't mean you need to syncronize you document flow with someone elses. As long as you have accurate records of what was delivered and what was invoiced you're books should be in order.

If you're determined to track these numbers, keep in mind that you still want your system to reflect the order in which you receive/create the information. If an invoice comes in after a delivery and each invoice line has a delivery date and a PO reference, add those respective fields to your supplier invoice section, but leave the pick slip you already created the way it is; there's no point in going back and editing the pick slip to reference the PO anyway, because in this scenario the PO's are meaningless as the PO lines don't match anything that was delivered or invoiced.

Whatever you do, avoid changing values on PO lines that have already been validated. I wouldn't worry about deliveries matching POs 1:1 because you're not using POs the way they were meant to be used in the first place (read: en.wikipedia.org/wiki/Purchase_order).

Hope this helps.