Caching popular files at mobile users' equipment and offloading them among users with device-to-device (D2D) communications can boost spectrum efficiency and alleviate the burden of backhaul links. However, it is difficult to prompt mobile users to share their resources voluntarily due to not only the selfishness but also limited battery lifetime. To enjoy the benefits brought by caching at the mobile user side and consider the corresponding energy consumption, in this paper we first propose a threshold-based energy saving (TES) strategy for mobile users. The mobile-user-determine-TES (MUDT) scheme is proposed for the network operator to offload data with a green incentive mechanism. Furthermore, the network-operator-determine-TES (NODT) scheme is proposed as a comparison scheme to evaluate the performance of the MUDT scheme. To encourage mobile users to participate in content sharing, VCG-based reverse auction is utilized as the incentive mechanism for both the MUDT scheme and the NODT scheme. Simulation results not only validate the property of the incentive mechanism such as truthfulness and individual rationality, but also verify the advantages of the proposed MUDT scheme over other benchmark schemes.