When is zakat due on the proceeds from the sale of land not designated for trade, given the existence of another sum of money intended for construction? Should construction expenses be considered a deduction from this amount, or should one wait for the customary Ramadan hawl (lunar year) to elapse before paying zakat?

The money obtained from selling land is not subject to zakat until a full year has passed. If a year passes and some of this money remains, its zakat becomes immediately due, and it is not permissible to delay its payment until the next Ramadan. Whatever was spent from this money before the year elapsed is not subject to zakat. However, whatever remains in your possession after a year has passed is subject to zakat, even if it is saved for construction or other purposes.