Hi Harald,
It could be that the field currency is maintained as mandatory for any of the component - AC/PC/RM.
As RM make the user of currency for calculations so its chances that its maintained as mandotory for atleast that component.
You can check the same in the view cluster (transaction SM34) cluster is GRFNVC_FLD.
Select Entity as ORGUNIT and click on left side "Field Status Configuration".
There check the field ID "OU_CURRENCY" and its status for different components.
In such case if you want to make that field as optional for PC/AC, its always suggested to create
different organizatinal views for different components.
SPRO -> Governance, Risk and Compliance -> Shared Master Data Settings -> Maintain Organization Views -> Maintain Organization Views Configurations.
Hope it helps.
Regards,
Silky