Consider a mobile ad hoc network where the nodes belong to different authorities. The nodes must be given incentives to spend their resources (battery power and transmission bandwidth) in forwarding packets that originate at nodes belonging to another authority. This can be done by assigning a credit balance to each node: when a node acts as an originating node it uses its credits to pay for the costs of sending its own traffic; when a node acts as a transit node it earns credits by forwarding traffic from other nodes.This paper presents a credit-based incentive scheme which assists nodes when they lack the credits necessary to transmit their data. An essential part of the credit-based scheme is a decentralised credit redistribution mechanism to destroy (create) credit at nodes that are over (under) supplied with credit.The goal of this paper is to investigate the effect of various fraudulent activities on the performance of the credit-based incentive scheme. We show that the credit-based scheme is inherently robust with respect to a number of fraudulent activities, provided the nodes behave in a rational manner. We show that credit redistribution is the main reason why the credit-based incentive scheme is relatively immune to fraud.