Abstract: There is described herein a method of conducting a mobile transaction, comprising: a card processor server receiving an authorisation message from an application programming interface (API) running on a sending institution server, the authorisation message comprising a push code indicating that the transaction should be processed immediately; and the card processor server transmitting a guarantee message to an API running on a receiving institution server, the guarantee message comprising an urgency code indicating that funds should be posted to a merchant's account immediately. There is further described a system comprising a card processor server, an API running on a sending institution server, an API running on a receiving institution server, a consumer mobile user device and a merchant user device, the card processor server being configured to perform the aforementioned method.