If I remember right, and it's been quite a while since I was on RR, but I believe it's optional to reduce on hand or not without printing the invoice. It can be set either way.
I kind of remember going at it with RR support on why my parts weren't coming up to reorder when they were on an open invoice and not picked up yet by the customer. They changed something to make it happen. At the time, I trusted that the customers we had would show up but I think it's turned off by default in case customers never come in.
I didn't want to wait 3 days for a part to reorder when it was being held for someone that I knew would show up but not for 3 days.