[ibr-031]-Allowance (ibg-20) MUST have an allowance amount (ibt-092).
ibr-031
cac:AllowanceCharge[cbc:ChargeIndicator = false()]
((exists(cbc:Amount) and not(ancestor::cac:InvoiceLine | ancestor::cac:CreditNoteLine))) or (ancestor::cac:InvoiceLine | ancestor::cac:CreditNoteLine)