Is zakat obligatory on an amount paid as a down payment for the purchase of a residential apartment, then refunded after the company's failure to build it?

If the sale is concluded, you are not obligated to pay zakat on the money for the period it was out of your possession, even if it returns to you later. Rather, you would start a new hawl (lunar year) from the day it returns to you. Zakat becomes obligatory on the company if the wealth reaches the nisab, because it became its property after the contract.