Is the vow of one who vowed to build a mosque on the ground floor of his property waived if he experiences financial hardship and sells the land, and is Zakat obligatory on the selling price of the land or on its purchase price?

You are not permitted to sell the land for which you made a vow to build a mosque on its ground floor and a residence above it, as an act of gratitude to Allah. This is because you have not yet despaired of being able to build, and you must wait until it becomes easy for you to do so and fulfill your vow. There is no zakat due on it because it is not for trade.