بسم الله الرحمن الرحيم
If after defining the length and width of the plot the dealing was done on fixed rate then you became the owner of the plot (though still it has not come into your possession) and the remaining amount shall be due on you. Whatsoever gold, silver, cash, business goods are in your possession, you should evaluate their total value and shall deduct the said debt from the total value and then pay the zakah of the remaining amount. But if you have not bought the plot with the intention of selling then no zakah shall be wajib on it.
(2) If the rate was fixed and you fixed the land and finalized the dealing (it was not mere promise) then you became the owner of the land and you shall have to pay the 90% amount which is due on you as it was mentioned above.
Allah knows Best!
Darul Uloom Deoband