It is impermissible to sell a plot of land or any other object except if it is in one's possession at the time of the sale. The Prophet (peace be upon him) said:
Do not sell what you do not possess.
The down payment is evidence to the finalization of the sale, and therefore it is impermissible in this case.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.