How is the Zakat calculated for a plot of land that was sold six days before its hawl (Zakat year) was due, and its price was received? Is the Zakat on the money immediately obligatory, or after a full hawl has passed on the money?

It is permissible to sell wealth before the hawl (one year's possession) is due, as long as the intention is not to evade zakat. If the wealth consists of trade goods, they should be sold, and their value added to other assets to complete the nisab (minimum taxable amount). Whoever sells trade goods before zakat is due on them must pay zakat on the wealth resulting from the sale when the hawl is completed.