How to reference a UNIQUE KEY CONSTRAINT in parent table as a foreign key in child table?
Thanks to ypercube, 

I had to drop the parent table "department", recreate it and then use ALTER TABLE employee ADD CONSTRAINT fk_manager_unique FOREIGN KEY (manager_id) REFERENCES department (manager_id); everything worked just fine and the foreign key reference to the unique manager_id key were finally created. Note that I had to change the name of the constraint "manager_unique" in the second table because of duplication issues. 

Thanks again!