You can sell land that is currently leased, even with a long-term lease still in effect. The key is that the new owner (purchaser) must agree to honour the existing lease terms and conditions for the remaining period. While you retain ownership of the land, the lessee has rights regarding any structures they've built. It's advisable to consult with a local lawyer and ensure the sale deed clearly outlines the situation regarding the superstructure and the continuation of the lease.