I have created a topic out of my previous comment.
How is having a line item custom field going to reduce human mistakes, aren’t you manually going to type into the field? With a custom line item field, you will have something like a separate column to list the request numbers on the form but it won’t reduce human error. A requisition workflow could be the solution as it will give a reference number to every requisition and show it on the Inventory Transfer form.
As mentioned in the topic Requisition Workflow In Manager - ideas - Manager Forum