If the reality is as you have mentioned, there is no harm in taking the
additional amount of money, as the buyers paid it out of their good will as a tip for you. But if you favored them and sold them the land while there was a higher price, it would be impermissible to take this addition, as it would be the price of favoring them.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.