Does zakat apply to an amount of fifty thousand Egyptian pounds paid as a down payment for a plot of land purchased in installments, upon which a year has passed, knowing that its purpose is to prepare for my daughter's marriage? And if it is obligatory, what is its amount?

Zakat is not obligatory on the amount paid (fifty thousand) because it has left the possession of its owner. Zakat is obligatory on land if it is prepared for sale (trade Zakat), and it is valued annually, and a quarter of a tenth of its value is to be paid. The Zakat year begins from the time the land's price is acquired, not from the time it was purchased. However, if it is for the purpose of housing or rent, or if the owner is hesitant, then trade Zakat is not obligatory on it. Debt does not affect Zakat.