How is the zakat calculated for a sum of money that was used to purchase two carats of land, given that the original intention of the purchase is not remembered, and the intention for using the land changed later?

You must intend the land for trade when you acquire it for it to be considered a trade asset subject to Zakat; otherwise, no Zakat is due on it. If you purchased land with the intention of trading (the third plot) in the seventh month of 2017, you must appraise it upon the completion of its Hijri year and pay two and a half percent of its value, combining it with any money or other trade assets you possess. As for debts, there is a difference of opinion regarding deducting them from the Zakat-eligible wealth. If you sell the land before the year is complete and purchase another plot with its proceeds, without the intention of trading, then no Zakat is due on it. If you sell land and pay off your debt, Zakat is obligatory on the remaining money if a year has passed over it.