I think you need to first add department_id to purchase.order model, then add (through related/function) the department's manager_id as well then filter by that.  The department_id can be a related/functional field but I think it is saver to put it as a normal field that will be defaulted by the requestor's department.  This way, the purchase will remain in the same department although the requestor has moved to another department.

Thanks Ivan,

Ok for adding field department_id to purchase.order model.
If I understand, I get the department’s manager_id when user make his purchase, and I write it into the new field ?
But how can I get this ? It does not seem to have rel between user and employee.
Could you help me for this function ?