In standard systems these fields (WGRU1 and WGRU2) are not editable.
Have you actually tried to find the solution on your own?
If you navigate to SPRO->Sales and Distribution->System Modifications, there is documentation on the various ways to influence the default system behavior in SD. In its sub-node User Exits->User Exits in Sales, they explain MV45AFZZ - USEREXIT_FIELD_MODIFICATION, which is how you can make fields editable, grayed out or hidden in sales documents.
Read the documentation on MV45AFZZ and discuss the changes with your developer.