Hello Gerlof,
After 12s2 goes live, you will be able to assign a user to a ne wuser group called "Close Order" and that will enable the user to close or reopen the POs (only the POs that are created by him/her). You could find the recorded demo of this feature in our connect pages. This user group is a small part of the larger feature called "Automatic and Bulk Closure of Purchase Orders".
Regards,
Deepak Sanghi
Ariba