Thank you Wayne for your input. I like the idea of tracking backorders too. This customer has lots of master orders, and their inventory is generally very accurate. This issue, where the quantity remaining to ship on the master order is incorrect, only occurs once in a while. It occurs enough that they are concerned, and want to know if either there is a bug in the software, or if they are doing something in the process incorrectly.
This was an ROR change to me about three years ago. I upgraded them from 3.74 two years ago and they are currently on 2017. They don't use shipping data entry. Picking tickets are returned to the office for invoice data entry. The inaccuracy occurs when, occasionally, there is a difference in quantity available and what is picked.
I have a telephone conference / remote access session with them tomorrow to review this issue. I am going to find out why they never backorder, and why they aren't using shipping data entry.