Sometimes the Purchase Orders and the actual Bills do not match. However, there are several solutions you can use to fix this issue:
Ignore the fact that the PO and the Bill are different. You still have an option to Reject or Approve the Bill within ApprovalMax.
You can fix the PO in Xero. Just edit the PO so it will match the Bill.
Cancel the PO and create a new one which will match the Bill you have received.
You may also issue a new PO for the price difference in ApprovalMax.