The cognitive framework, which fosters an efficient use of the channel resource by granting access to smart and adaptive terminals, has been recently integrated with the cooperative paradigm. Accordingly, following a spectrum-leasing approach, licensed (primary) users may let unlicensed (secondary) users access the owned bandwidth in exchange for an increased performance via cooperation. Nevertheless, assuming that secondary transmission is limited to idle primary slots, as the traffic from the primary sources increases, implementation of spectrum leasing via cooperation becomes more challenging. In fact, the opportunities for secondary transmission reduce and the secondary users may find spectrum leasing not advantageous. In this paper, a solution is proposed that exploits fading temporal correlation and retransmission-based error control in order to increase secondary throughput even in high primary traffic conditions. Focusing on a simple network with a primary and a secondary source transmitting to a common destination, it is shown that the primary source may actively leave part of its channel resource to the secondary source without significant performance loss while greatly improving secondary throughput.