In case the Amount due for payment (BT-115) is positive, either the Payment due date (BT-9) or the Payment terms (BT-20) shall be present.
/ubl:Invoice/cac:LegalMonetaryTotal/cbc:PayableAmount((. > 0) and (exists(//cbc:DueDate) or exists(//cac:PaymentTerms/cbc:Note))) or (. <= 0)