Does zakat become obligatory upon a man who owns a restaurant encumbered by debt, has an amount for working capital, three uninhabitable houses, and pays installments for his daughters' education and housing?

Debt prevents Zakat (definition) on internal wealth and trade goods if it depleges the wealth or reduces it below the nisab (definition), provided there are no other non-Zakat-eligible assets that can be used to offset the debt. Since the value of the houses you own equals or exceeds the debt, you are required to pay Zakat on the cash amount you possess, which is thirty thousand Euros, and you should pay a quarter of its tenth (2.5%).