Matching additional Purchase invoice
There are two ways of doing this depending on the information you ultimately want.
If you dont mind NOT being able to see the cost against the purchase order
You can expense the invoice off directly to the P&L by approving the invoice without matching. Baan may take you to the matching screen but exit this and expense the invoice.
If you want to see the costs against an order
You will need to add an extra line in the purchase order and book the invoice against this. You will most likely need to use a cost item or service item. This will mean the P&L will be expensed as soon as it is received, which is similar to the above but obviously you can see what the costs for that order were.
A slight word of caution if you use cost price calculation: If an order situation such as you describe happens and the part ordered is used for cost price calculation your costings may be inaccurate (depending on calculation priority). You may need to consider splitting the order into two or more lines at the time of placing the order to reduce the possibility of inaccuracy.