posting of sales invoice

hi folks,

how is it possible that a person with warehouse permission able to post sales invoice?
how testing done is summarized as below, first 2 phase is to confirm identity of user whom post the sales invoice.

phase 3 is to simulate the error using the existing permission of warehouse staffs


PHASE I: (verify the person who did the posting of sales invoice)
-navigate into G/L entries
-check created by = blank, user id= warehouse staffs
-navigate into Value entries
-check created by = blank, user id= warehouse staffs
-navigate into Detailed Customer entries
-check created by = blank, user id= warehouse staffs

PHASE II: (verify the person permission being amended before)
-Filter by Table No: 2000000003 (member of)
-Filter by Primary Key Field 1 Value: warehouse staffs
-Filter by Primary Key Field 2 Value: Sales Permission

PHASE III: (using warehouse staffs permission to try and do sales invoice posting)
-restore database into my local as SA
-set the same permission as SA
-create sales invoice as SA
-post sales invoice as warehouse staffs login
-RESULT: hit permision error of inserting into sales invoice table

ps: by the way i am using an unstructured warehouse.

if based on the g/l entries it should be the warehouse staffs who is posting the sales invoice, however when i tried
to login as the warehouse staffs credentials and post, i am hit with error of not having permission to insert into sales invoice table..