How you are specifying that XXX-XX-X WBS is relevant to ABC department and so on? Are you using any custom field like 'Owner code' etc. to specify it?
Through std. customizing its not possible but through development it can be done as follows:
If you are maintaining department/owner code field n project structure for wbs then use this department-WBS field combination to validate it during PO, you need to maintain 'Department' field as a mandatory field in PO master (check whether it's custom field or std. in PO,i think it would be a custom one).
Once department field appears in PO that validate it from 'department-WBS' combination of project structure (use these fields from table PRPS) and pop-up warning/error message accordingly. You would require stand. user-exit for it in PO search for it in MM forum.
And if, you are not maintaining department/owner code field in WBS master then maintain it in a separate table (WBS->department) and validate in the same way as explained above.
Thanks.