Is zakat obligatory on land purchased with the intention of building and then selling, and on the remaining amount from the sale of a property whose profit zakat has already been paid?

Zakat is obligatory on land prepared for trade and construction, then sale. Regarding goods, it is on the money with which you purchased them.

If you purchase real estate with the intention of trading it, Zakat is due on it. It is not sufficient to pay Zakat only on its profits; rather, Zakat must be paid on the principal capital and the profits together if a year has passed since the intention of trading was made.

However, if you purchase real estate for dwelling or renting, there is no Zakat on it unless you later intend to trade it. In that case, Zakat becomes obligatory if a year has passed on the money available from the rent and it reaches the Zakat threshold.