The 638 acres (2.58 km2) of land [We Build the Wall] built on is part of farmland that belongs to Neuhaus and Sons, and the wall added over $20 million in taxable land improvement, increasing the tax burden by 75 times. In January 2020, Fisher Industries started a lease-purchase agreement with Neuhaus and Sons for the land under the wall, but had not completed the ownership transfer by their court hearing on December 12, 2020, citing a problematic land survey by Fisher. Fisher's attorney, Mark Courtois, was hopeful the US government would become owners of both the wall and land. U.S. Customs and Border Protection (CBP) Public Affairs Officer Thomas Gresback said that the wall was privately paid for and on private property, and CBP does not have anything to do with the project. CBP is constructing its RGV-03 project wall outside the floodplain 0.3 miles (0.48 km) away.[66] As of July 2021, the property had been reassessed at 100 times its original value, and Fisher was hoping to sell a 3-mile section of wall (4.8 km) that had cost $30 million to build.[67]
That land used to have nothin much on it, so it was cheap land, and the property taxes were low.
30 million was spent on building the private wall. I don't think this money came from the land owner. (This is the crowd funded Trump Wall, right?)
With the fancy new wall on it, the property is now appraised at around 20 million (or maybe whatever it was plus 20 million). Whether it's 75 times or 100 times what it was before is not super important. The point is it went from not much to quite a bit.
Property taxes went up in accordance with the new valuation.
Property owner is probably not wealthy enough to pay the taxes, so big picture he can't continue to own the property.