we have an Open Invoice for a Retailer from back in May (period is closed). We still need to collect money from them but since the order went out in two fulfillments, they need invoices for each fulfillment instead of one invoice for the whole order. How do I go about the old invoice? I can't void it because it sits in a closed period. Creating a journal entry and applying it makes the invoice paid and the sales order billed so that won't work.

You dont need to void it, just record events as they happen - in this case you need to credit out the old invoice (with a credit memo specifically), closing that liability, then create 2 new ones and collect on those. If not collecting at all you write it off vs e.g. a bad debt / loss account

@PWN If we create the 2 new invoices though, how do we get it related to the original sales order? The credit memo applied makes it look like the order was fully billed