1. A method for use in a system that includes a server, a supplier system, a buyer system, a first finance system, and a second finance system, wherein the supplier system is used by a supplier, the buyer system is used by a buyer, the first finance system manages a bank account of the buyer, the second finance system manages a bank account of the supplier, and the supplier system, the buyer system, the first finance system, the second finance system, and the server are connected to and communicate via a communication network, wherein the method comprises: the server receiving an electronic invoice from the supplier system via the communication network, wherein the electronic invoice relates to a transaction between the supplier and the buyer, wherein the electronic invoice includes a unique identification code, wherein the unique identification code identifies the electronic invoice from a plurality of electronic invoices issued to the buyer by the supplier, and wherein the unique identification code is generated by the supplier system; the server registering the electronic invoice in a database; the server transmitting information that defines a Graphical User Interface (GUI) screen to the buyer system via the communication network, wherein the GUI screen is for showing contents of the electronic invoice registered in the database and for allowing the buyer to approve the contents of the electronic invoice; the server, in response to an approval of the electronic invoice performed by the buyer using the GUI screen, creating a transfer request message, wherein the transfer request message includes the unique identification code included in the electronic invoice; and the server transmitting the transfer request message to the first finance system via the communication network, such that, following reception of the transfer request message at the first finance system: the first finance system and the second finance system are able, using the unique identification code, to perform a deposit of the amount of the electronic invoice from the bank account of the buyer into the bank account of the supplier; the second finance system is able, using the unique identification code, to send an electronic deposit/withdrawal statement to the server or the supplier system, wherein the electronic deposit/withdrawal statement describes that the deposit from the bank account of the buyer into the bank account of the supplier has been made, and wherein the electronic deposit/withdrawal statement includes the unique identification code; and at least one of the supplier system and the server is able to settle the electronic invoice using the unique identification code.