Declaring income from sale of plot in IT returns

Hi,

I am planning to sell my plot. The buyer is willing to show only the Government Rate of the plot in the Sale deed and wants to pay the remaining amount (i.e., Market Rate minus Government Rate) by cash.  He is doing this so as to save on the registration costs.
 
Can I ask the buyer to do online transfer of the remaining amount (instead of me accepting it by cash).? If so, can I declare only the Govt Rate (mentioned in Sale Deed) while filing IT returns.? 

Or should I declare the full Market Rate in the IT returns (even though the Sale Deed mentions the Govt Rate value).?

In a few months, this whole money will be spent on the construction of a house on my another plot.

regards,
Raghunath