Ah I see
If you can or cant buy an area (they are called farmlands by the game) is set in the farmlands.xml file in the map. This file is usually located in the same folder as the map.i3d file. In a mod that would usually be modName/map/farmlands.xml
In the farmlands.xml file each farmland (or area) is defined with a number (eg farmland 1), this is the same number that you see ingame when you buy an area. Now you will need to know which number corresponds to the area that you are interested in. As it currently cant be bought it is likely that it will be the same farmland as everything else that cant be bought on the map. If you want to you can change this, but thats up to you.
To find out which number the farmland you are interested in is, open the map in giants editor. In the terrain editing panel under "Info Layer Painting" in the "Info Layer" dropdown select "farmland", then uncheck the checkbox "Show Unmasked" and enter terrain info layer paint mod, by clicking the icon with the little blue "i" symbol in the toolbar at the top. Now in the "Lands" dropdown select one of the farmlands (eg "Farmland 1"). Now the area that is farmland number 1 is highlighted.
You can now go through and select one farmland after the other in the "Lands" dropdown until the area you are interested in is highlighted. Now simply looking at the "Lands" dropdown will show you which number the farmland is.
Close giants editor without saving (it will ask you if you want to save changes, select "no") and open the farmlands.xml file in a text editor.
Find the line that contains
where number is the number of the farmland you are interested in (the one you just found in giants editor).
If this line contains
then delete this bit and save the file.
If you cant find a line with the correct number, copy a different line and change the number in the copied line to be the one you are interested in. Make sure the line does not contain the above mentioned string.
Save the farmlands.xml file.
The next time you load the map you should be able to buy the farmland.
If you are on a multiplayer server, you will have to change this both on your client and on the server and everyone else who wants to join the server will also need the modified version of the map.
Also, if you update the map through the ingame "Downloadable Content" menu, you will have to redo the changes, because it will overwrite your modified version of the map.