How is the zakat calculated for land sold at a loss after it had been designated for earning and trade, and is zakat obligatory for past years during which its market value was unknown, and is zakat waived upon selling at a loss due to pressing need?

If the land is designated for trade and investment, then trade zakat (definition) becomes obligatory on it. Zakat must be paid for all past years, and its value is assessed at the end of each hawl (lunar year). One-quarter of one-tenth (2.5%) of its value is to be paid each year, even if it is sold at a loss. However, if the intention behind building on it is for rent, with the intention of selling it completely abandoned, then trade zakat is not obligatory on it.