Multipath mobile data offloading technologies allow user equipments (UEs) to aggregate cellular network resources and WiFi network resources in order to increase the total effective bandwidth. The IETF recently proposed Multiple TCP (MPTCP) as an extension of the standard TCP protocol to enable UEs to deliver data streams across multiple connections simultaneously. By combining multipath mobile data offloading with MPTCP, operators have the ability to adjust the cellular bandwidth assigned to the users in accordance with their WiFi connection status, thereby saving valuable LTE resources while simultaneously ensuring the quality of service (QoS) of the transmission. Accordingly, this study proposes a dynamic bandwidth reallocation scheme which reduces the cellular bandwidth allocated to a UE when WiFi access is available and reassigns the saved LTE bandwidth to other UEs with no WiFi connection. The effectiveness of the proposed scheme is demonstrated by means of numerical simulations. It is shown that the proposed method successfully reduces both the deadline miss ratio of deadline assurance services and the amount of idle LTE bandwidth.