Is zakat obligatory on a plot of land that was purchased with the intention of building on it, given the possibility of selling it and buying a better one? And is zakat obligatory on what is deducted from one's salary, and how is it calculated? And is it permissible for an employee who stole money from his company to return it by purchasing items for the company with the stolen money if he is asked to do so?

If you bought land to build upon it, or with the aim of selling it when a better plot becomes available, there is no zakat (definition) on it, because it is not considered an article of trade. For more information about zakat on land, please refer to Fatwa (definition) no. 84293. Regarding zakat on salary, please refer to Fatwa no. 156249 and what it references. As for your last question, please submit it separately.