Two examples from my European Country, where the practice is spread and people believe they are smart but are really not.
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a lot of people did it here, especially in the 80s-90s. Then we started building high speed 4 lanes highways and high speed trains. You know how much the State paid for the land used to build those infrastructures?
EXACTLY!
The nominal values, so basically people got compensated for a tenth of the actual price.
The State also explained publicly that most of the land in straight flat paths, was chosen using their market value. If it's worthless, nobody will protest, on the contrary, they will be happy that someone is buying it for a price.
Some of these people were so stupid that sued the State. Of course the State won and also discovered the fraud, collecting even more money in the form of fines.
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Recently a campaign of subsidies to improve energy efficiency of buildings was launched. The benefits are up to 110% of the total costs. Yep, 110%.
So my parents, who always did everything by the book, and got called foolish by their neighbors for years, got for free a 45k photovoltaic system and the renewal of the shed, while most of the requests were rejected because various irregularities were in place. Which again did the government a tool to investigate who committed a fraud in the past and fine them.
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Personal advice: unless you're Bezos level rich, don't try to outmsart the government. Chances are your children will have to pay for your assholeness.
So someone might officially pay 20.000€ for a piece of land, pay the taxes and fees based on that price, but pay the remaining 60.000€ in cash.
Obviously this is illegal, but the notaries are in on it.