Define the normal path
Specify the validation, record updates and output for an ordinary transaction. That becomes your baseline test.
Make exceptions explicit
Insufficient stock, missing fields and an unexpected amount should pause the work with a useful reason. Do not let errors silently become downstream records.
Keep decisions attached
Record what was approved and when. Review the same input snapshot so that a later edit cannot change an earlier decision.
Test the boundaries
Try an amount exactly at the threshold, one above it, a duplicate event and a failed prerequisite. The workflow should behave predictably in each case.