In an IoT cloud, the message broker service allows point to multi-point communication between IoT devices and applications. The service is governed by Service Level Agreements (SLA) that specify, among other requirements, the volume of messages served during an enforcement period. For simplicity, current SLAs do not provide detailed information about message arrival patterns, making enforcement of the SLA a difficult problem for the providers. In this paper, we propose a new, two-step SLA by introducing sub-periods for measurement and control within the total enforcement period. Our proposed SLA retains the simplicity of the current SLA and provides additional controls for the providers to enforce it. We present the conformance of the new SLA as a resource allocation problem for cloud providers and propose a buffering, scheduling and rate limiting mechanism to enforce it. We verify that the solution achieves conformance, analyze the tradeoffs of the solution and evaluate via simulation the effects of system parameters such as capacity, number of sub-periods and enforcement period.