Does zakat become due on an amount refunded after one year, given that it was paid to purchase land, and noting that it was taken as a loan?

The money has not left your possession as long as the contract has not been finalized between the agent and the landowner. Therefore, zakat (definition) is obligatory upon you for it at the end of the year, if it reaches the nisab (definition) by combining it with any other zakatable wealth you possess. It is more prudent not to deduct any debts you owe from the money on which zakat has become due, in order to avoid disagreement among scholars.