Attach Invoice to Sales Order

How do I go about attaching an invoice to a sales order? It appears that our bookkeeper inadvertently created an invoice that billed our customer 1 qty of an item. The Sales Order that she was working off of was 3 qty. However, she evidently created the invoice rather than using the Bill Sales Order menu. This has caused us to almost bill our customer twice, fortunately, we caught it. The second invoice was deleted without problems. I have found no way to attach outside of billing the sales order which turns it into an invoice, then manually changing the