Is Zakat obligatory on the entire saved amount, or only on the amount that exceeds half the remaining price of the land?

The remaining money from the price of the land is a debt upon you to its owners. Zakat is not obligatory on your current money unless you possess surplus funds beyond your needs that are equivalent to the value of the debt; in that case, you pay Zakat on this remaining amount. As for the other money you acquired, you pay Zakat on it when a full year has passed since you possessed it, provided it reaches the Nisab either by itself or when combined with other funds.