Is Zakat obligatory on a sum of money returned to its owner after it was withheld for the purchase of a residential apartment whose construction was not completed, and this for the entire period of its withholding?

If the money paid to the company was the price of the apartment and the sale was concluded, then there is no zakat due on you for it, and the zakat is due on the executing company. However, if the money was a deposit and has not been transferred to the company, then zakat is due on you throughout the reservation period.