Is zakat immediately due on the proceeds of a sold agricultural land upon their receipt?

There is no Zakat due on you for the price of the sold land unless a full year has passed while it is a Nisab, provided you are not a merchant dealing in it. If you are a merchant dealing in it, then Zakat becomes obligatory when a full year has passed, whether it has been sold or not. The land is appraised if it has not been sold, in order to pay Zakat.