Is zakat obligatory on a plot of land designated for sale, knowing that it was originally intended for building a residence, and was only sold after four months of being offered for sale? And when does the calculation of zakat begin: from the intention of building or from the intention of selling? And what is the amount of zakat?

Zakat (definition) is not obligatory on land until it is sold. Upon its sale, a new year (hawl) begins for its price. If a hawl passes on its price, and it reaches the nisab (definition) either by itself or combined with other money or trade goods, then zakat becomes obligatory on it. The nisab is the equivalent of 85 grams of pure gold or 595 grams of silver, and one-quarter of one-tenth (2.5%) must be given out.