Is Zakat obligatory on the entire amount saved to buy an apartment, or on the remaining amount after deducting the due payments, knowing that the amount has reached the Nisab of Zakat and a Hawl (full lunar year) has passed over it, and the apartment has not yet been received or the contract signed?

If the price of the apartment is a debt you owe to the government, you can deduct the value of the debt from the money you have accumulated, provided that you do not possess other funds exceeding your needs that could cover the debt. After deducting the debt, you pay zakat (definition) on the remainder if it does not fall below the nisab (definition), and you give out a quarter of a tenth (2.5%) of it. In summary, you only deduct the necessary debt, whether it is the full price of the apartment or merely the reservation fee.