I suspect you're thinking about the deposit and withdrawal amounts ($10,000) that trigger federal reporting requirements at banks. There's no law prohibiting carrying cash. However, civil asset forfeiture permits law enforcement to seize any currency that they have reasonable suspicion to believe is the product of or intended to fund illegal activity. That's an extremely low standard. The only way to get it back for the owner to prove by a preponderance of the evidence that the money is legitimate. In other words, the burden gets shifted to the person from whom the money was stolen, and he has to meet an even higher burden of proof than applied when the money was seized. Laughable.
More on the reporting requirements--Again, it's not just about your deposits; it's your withdrawals too. If you take $10,000 or more of your own money out of the bank, you're required to explain why, and that information goes to the federal government. That's what bit Dennis Hastert in the ***.
Completely agree. It's actually more than just the Fourth Amendment. Fourth deals with searches and seizures. Fifth discusses the prohibition on the federal government taking life, liberty, or property without due process. The Fourteenth imposes the same due process requirement on state governments.
Asset forfeiture is unconstitutional across the board. It's like a modern day Sheriff of Nottingham stealing from the people to enrich himself and his men. A true embarrassment for our country.