Purchase Order Approval Workflow

We have created a custom purchase order approval workflow which included two level of approval (budget holder & senior budget holder approval). The budget holder & senior budget holder approvers have been assigned 'Employee Centre' roles. To be able to view the purchase order the workflow has to change the employee field allowing the user to view another employee's purchase order, since they are not their supervisor. The problem is whenever saving or approving the purchase order the user receive the notice 'Permission Violation: You need a higher level of the 'Transactions -> Purchase Order' permission to access this page.