Is zakat due on money that was used to purchase a plot of land before the zakat due date, but the contract signing procedures were not completed until the month of Ramadan?

What you did by paying the Zakat (definition) before spending the money is correct, because Zakat becomes obligatory on all Zakatable wealth that has reached the Nisab (definition) and a full year has passed over it. After purchasing the plot of land, Zakat is no longer obligatory on it once you own it, and legal procedures are not considered. This applies if it is for personal use (Qunya), in which case there is no Zakat on it by scholarly consensus, based on the Prophet's (peace be upon him) saying: "There is no Sadaqa (Zakat) on a Muslim for his slave or his horse." However, if it is for trade, then its Zakat becomes obligatory every time a year passes over its original value.