The issue is that there is no legal contract. If I pay someone $400k in cash for them to build a building, but they don't do it, I cannot sue them to get the mo

The issue is that there is no legal contract. If I pay someone $400k in cash for them to build a building, but they don't do it, I cannot sue them to get the money back since it was illegal to pay that way anyways. Alternatively, if the building was built first but they were never paid then the builders cannot demand payment through court.