How is Zakat on money calculated for an amount paid as a down payment for a flat, then later refunded after the cancellation of the contract, and is Zakat due for past years, or only from the date the amount was refunded?

The money paid as the price for an apartment is not subject to zakat (definition) for the past years. If the contract is terminated and the money is recovered, zakat becomes due on it after a new year (hawl) from the date of its recovery.