Is zakat obligatory on a plot of land designated for trade, if a year has passed without it being sold or its market value being known? Is it permissible to pay zakat on the amount paid for it, knowing that zakat was paid on it before the purchase? And how is zakat calculated on money earned monthly from working abroad if a year passes while one is on vacation?

Zakat (definition) is obligatory on land purchased with the intention of trade. Its hawl (Zakat year) is tied to the money used to purchase it. The land is appraised at its current value at the completion of the hawl, and a quarter of a tenth (2.5%) of its value is to be given as Zakat.

Zakat must be paid for every hawl that has passed on the land, even if delayed for years. If you are unsure, you should use your best judgment in estimating the value and the number of years.

As for salary and other funds that do not result from profit generated by your existing wealth, Zakat is not obligatory on them before a full hawl has passed since their acquisition. However, it is permissible to pay their Zakat in advance along with the Zakat on previous wealth.