In an IoT cloud, the message broker service offered to customers allows customer's devices and applications to connect and interact with cloud applications and other devices. 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, SLAs do not take into account detailed information about message arrival patterns or service times, making enforcement of the SLA a challenging problem for the providers. In this paper, we propose a resource allocation mechanism (that uses buffering, scheduling and rate limiting) to meet the SLA. 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 and enforcement period.