All Praise to Allah Subhanahu wa Taala and blessings of him be on Prophet Muhammad peace be upon him his family and companions

Zakat is payable on trade commodities. For something to be considered a trade commodity, a clear and firm intention of it being so needs to be present at the time that good is purchased. If one purchased the good without the intention of it being a trade commodity, Zakat would not be due upon it, even if the intention was changed later on. [Sarakhsi, Mabsut; Kasani, Bada`I al-Sana`I; Haskafi, Durr al-Mukhtar]

Thus, land bought with the primary intention of being a trade good at the time of purchase would be considered a zakatable-asset.You didnt primarily bought it for that purpose so zakat was not due but Now you have the amount so you need to give zakat on it and further if you buy a land it depends on your intention too.

> Sayyiduna Samura ibn Jundub (Allah be pleased with him) narrates that
> “the Messenger of Allah (Allah bless him & give him peace) used to
> command us to pay Zakat of those items that were for sale.” (Sunan Abu
> Dawud)

Source: http://seekershub.org/ans-blog/2012/04/30/zakat-on-investment-land/
Also see this http://islamqa.org/hanafi/daruliftaa/8561.

Allah Swt and His Messenger knows best.