Is zakat obligatory for the owner of a taxi office who bought cars through an Islamic bank, having paid part of their price and still owing installments, and if it is obligatory, what is its annual amount?

The taxi cabs themselves are not subject to Zakat (definition). However, Zakat is obligatory on their earnings if they reach the nisab (definition) and a full year has passed over them. The nisab is equivalent to the value of 85 grams of gold or 595 grams of silver, and the amount of Zakat to be paid is a quarter of a tenth (2.5%). The remaining installments on the vehicles do not invalidate the Zakat obligation if the wealth reaches the nisab, because it is presumed that there is other wealth equivalent to the debt, thus not affecting the obligation of Zakat.