To try and simplify things....

Fact: The carrying of any amount of currency is not illegal with in the United States.
Fact: The transporting of any amount of currency within the United States and/or into or out of the United States is not illegal but you must report/fill out FinCen Form 105 if you are transporting currency (or a currency equivalent) either into or out of the United States if the amount is greater than $10,000.U.S. If you fail to report, you are in violation of the law and have to face the consequences.

That's all that is required-plane and simple

As to the source of the funds, structuring transactions and such, it is up to the entity* (i.e. bank, CBP) empowered by statute (i.e. the Bank Secrecy Act) to make that determination. Again-plane and simple


*but not the TSA and do not let them tell you otherwise