The rationale of this work is that the incumbent primary user (PU) of a cognitive radio (CR) system relays its traffic through the secondary user (SU) network for an enhanced throughput. In return, it rewards the SUs with an incentive time during which they can use the complete licensed bandwidth to transmit their own data, without the need for spectrum sensing. With the objective of maximizing the time incentive, we develop a MAC layer resource allocation strategy and a corresponding routing technique. A low-complexity decoupled cross-layer design is proposed to make the symbiotic relaying scheme practically workable. Simulation results are provided, which are indicative of the effectiveness of the proposed design.