Is zakat due on a piece of land that its owner intended to build upon or sell when needed, and he paid zakat on it for three years, then he was told that no zakat is due on it except when it is sold? So, is zakat due on it now after its sale?

Zakat is not obligatory on land unless it is intended for trade. When land is sold, zakat is not immediately due on its price. Instead, a new lunar year (hawl) begins for this money. If a full lunar year passes and the amount has not fallen below the nisab (minimum threshold), then zakat becomes obligatory at that point.