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A primary (licensed) link communicates in the presence of an interferer. A secondary (unlicensed) link is also active in the same band and can access the channel through a spectrum leasing (SL) mechanism, whereby the primary system grants transmission opportunities to the secondary link. This paper investigates the possibility that the secondary link gains access to the channel, to transmit its own data, by cooperating with the primary link via interference mitigation. Specifically, SL is enabled if the interfering signal is decoded by the secondary link and is forwarded to the primary link to allow for interference mitigation. The SL decision at the primary link hinges on whether the advantage accrued from interference mitigation by allowing secondary transmissions overcome the loss of spectral resources due to SL. This form of primary-secondary cooperation contrasts with previously proposed approaches to SL, whereby the secondary user gains credit by forwarding the primary packet, and not the knowledge of interference to the primary receiver (PR). A SL scheme is proposed that leverages the hybrid automatic repeat request (HARQ) retransmission processes at primary and interfering links. Numerical results demonstrate conditions under which the proposed approach based on interference forwarding outperforms more conventional techniques based on primary packet relaying.