Your sale of this land is not permissible and it is not a valid sale, (Part No. 13; Page No. 239) because you were not the owner of the land. You were not entitled to sell the land or give consent from its owner after you sold it. Hence, you have to repay the money, whether little or much, that you have taken from the purchasers.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.