Is Zakat obligatory on one who owns land for building and a taxi that generates 1000 EGP per month, while he pays 800 EGP in installments to banks?

The questioner did not specify the purpose of the land. If it is for dwelling or rent, there is no zakat (definition) on it. Zakat is due on the rent if it reaches the nisab (definition) and a year has passed over it. However, if it is for sale (for profit), zakat is obligatory on it, like trade goods. It is valued at the end of the year, and one-quarter of one-tenth of its value is to be paid.

Regarding debts, they prevent the obligation of zakat on hidden wealth and trade goods if the debt consumes all the wealth or reduces it below the nisab, provided there are no other non-zakat-eligible assets to offset the debt.

So, if the land is trade goods, and the installments consume the price of the land and the car's income or reduce it below the nisab, and you do not have other money to offset the debt, then no zakat is due on you. However, if the installments do not reduce the money below the nisab, and a year has passed over it while it is at the nisab, then zakat is due on the land and the car's income after deducting the value of the debt. A portion of the land can be sold to pay it. If the land is not trade goods, then no zakat is due on you.