Actually, our SAP works this way.
501040018 is the real SAP billing no. but we do not communicate outside. Only FI staff uses this number range.
501039898 is billing no. for Customs Invoice and we use this number to communicate with the customer too. When the customer transfers the money to us, the reference number must be the same as our declaration with Customs Dept.
Pls avoid this kind of duplication in future correspondence with the customer.