What is the legitimate, Sharia-compliant way to recover money from a piece of land that was sold at a building-land price, even though it is agricultural land, given that it is highly probable the buyer will build on it, and knowing that it is not suitable for any use except for construction?

The answer to most of this question has been provided in previous fatwas.

Regarding the zakat on land: If you bought it with the intention of selling it, zakat becomes obligatory on it as long as that intention persists. However, if you change your intention to holding it for personal use (acquisition) or if you become hesitant, then there is no zakat on it.

A condition for zakat to be obligatory on land is that it must have been purchased for trade and sale. The default status of land is for personal acquisition.

Since you were not firm in your intention to sell the land when you bought it (e.g., "I might build on it or save my money in it"), there is no zakat due on it. This is because mere hesitation about selling does not remove the land from its original status of acquisition.