Message

When the Supplier is Greek, the Invoice Id first segment must be a valid TIN Number and match either the Supplier's or the Tax Representative's Tin Number

Context
/ubl-invoice:Invoice/cbc:ID[$isGreekSender] | /ubl-creditnote:CreditNote/cbc:ID[$isGreekSender]
Test
string-length(normalize-space($IdSegments[1])) = 9 and u:TinVerification($IdSegments[1]) and ($IdSegments[1] = /*/cac:AccountingSupplierParty/cac:Party/cac:PartyTaxScheme[cac:TaxScheme/cbc:ID = 'VAT']/substring(cbc:CompanyID, 3, 9) or $IdSegments[1] = /*/cac:TaxRepresentativeParty/cac:PartyTaxScheme[cac:TaxScheme/cbc:ID = 'VAT']/substring(cbc:CompanyID, 3, 9) )