All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is only an obligation to pay the Zakah if the money reaches the Nisaab (the minimum amount liable for Zakah) and a whole lunar year elapses over it. The Zakah on the tradable items is based on the original money with which they were purchased. Hence, you should have paid the Zakah on the money when a whole lunar year had elapsed over the price at which you bought the land.
Moreover, in principle, Zakah should be paid on the capital money and on the profit altogether and you should not have paid the Zakah on the profit only. For more benefit, refer to Fatwa 133630.
Therefore, if the price of the land is in your possession until one lunar year has elapsed since the time you possessed it, then you are obligated to pay Zakah on all of it (both the purchase price and the profit), which is 2.5%. If you had spent it before a whole lunar year elapses, then you do not have to pay Zakah on it.
In any case, the money that you had paid with the intention of Zakah is sufficient for that amount of money (the profit of the sold land), so you do not have to pay it again, as you are acquitted from sin because it is permissible to pay the Zakah or part of it before its due time.
Allaah Knows best.