A computer-implemented method for electronic transaction message conversion to integrate a single-message scheme into a point of sale device, comprising: receiving an authorization request message from the POS device, the authorization request message comprising transaction data; simulating processing of the authorization request message by an authorization simulation module to: validate the authorization request message; store the transaction data in a transaction data database; generate a provisional authorization response message; send the provisional authorization response message to the POS device; receiving a capture message from the POS device, the capture message having been generated by the POS device in response to the provisional authorization response message; validating the capture message; translating the capture message to a composite message, the composite message being single-message system -compatible; and sending the composite message to a payment network; wherein the authorization request message and the capture message are dual-message system -compatible; and wherein translating the capture message to a composite message comprises: converting a first message type indicator of the capture message to a second message type indicator corresponding to the composite message; merging at least a portion of transaction data retrieved during the validation step with transaction clearing and settlement data of the capture message; and inserting the second message type indicator and the merged transaction and transaction clearing and settlement data in the composite message. receiving an authorization request message from the POS device, the authorization request message comprising transaction data; simulating processing of the authorization request message by an authorization simulation module to: validate the authorization request message; store the transaction data in a transaction data database; generate a provisional authorization response message; send the provisional authorization response message to the POS device; validate the authorization request message; store the transaction data in a transaction data database; generate a provisional authorization response message; send the provisional authorization response message to the POS device; receiving a capture message from the POS device, the capture message having been generated by the POS device in response to the provisional authorization response message; validating the capture message; translating the capture message to a composite message, the composite message being single-message system -compatible; and sending the composite message to a payment network; wherein the authorization request message and the capture message are dual-message system -compatible; and wherein translating the capture message to a composite message comprises: converting a first message type indicator of the capture message to a second message type indicator corresponding to the composite message; merging at least a portion of transaction data retrieved during the validation step with transaction clearing and settlement data of the capture message; and inserting the second message type indicator and the merged transaction and transaction clearing and settlement data in the composite message. converting a first message type indicator of the capture message to a second message type indicator corresponding to the composite message; merging at least a portion of transaction data retrieved during the validation step with transaction clearing and settlement data of the capture message; and inserting the second message type indicator and the merged transaction and transaction clearing and settlement data in the composite message.