Abstract: Remotely authenticating an invoice amount for a sales transaction with a merchant through a mobile device of a user using a USSD session initiated by a telecom service provider. The provider receives a transaction request of at least a mobile phone number and invoice amount from a POS machine at the merchant. The provider identifies the user from the mobile number and generates a one-time purchase ID associated with the invoice and sends the purchase ID to the user through the mobile device. The provider initiates an interactive USSD session with the user through a gateway and sends a menu with transaction details and financial account list on the user's mobile device within the session including multiple financial accounts registered to the user. The provider receives a password from the user through the session and deducts the invoice amount from a user's account and transfers the funds to the merchant.