Add additional Sales Order to Existing Invoice

When we have a completed sales order and invoice the customer, after the customer receives the invoice he find that we need to add an item on the invoice. We add the item on the sales order, so we can pick from inventory and fulfill the item, but we cannot get the item on the existing invoice. It want to create a separate invoice, but the customer need to submit a PO for the one invoice. I cannot delete the original invoice and create a new one, because it says we have linked transactions. What is the proper way of