Let's say I need to bring over $10,000 into the US and decide to use cash to save on transfer fees. What would happen at the border when I declare all that cash? Would CBP briefly question me on the source of funds or require some proof to confirm that the funds are from a legal source?
Are there any official guidelines on this matter?
I've done this once (doing all the right paperwork up front). They pulled us out for questioning: "where does the money come from" "why do you need this" "what are you planning do to with this", "why do you use cash", etc.
The amount of questioning will depend on how much you fit into any "suspicious" category otherwise. There is plenty of profiling going on there. Fortunately we were "mostly harmless" for everything else (white, middle class, small children it tow, from a "good" country).
You also need to be careful how this looks against your visa status. If you have a non-immigrant Visa or an ESTA, carrying lots of cash can be interpreted as "intent to immigrate", in which case they can turn you around and send you back.
Unfortunately customs and immigration officers in the US have a lot of leeway and discretion and not a lot of rules to keep them grounded. Could be harmless, could be lots of trouble, hard to predict.
You simply fill out a form at the border. See the official CBP page Currency / Monetary Instruments - Duty on money, checks, etc.
Travelers leaving or entering the U.S. are required to report negotiable monetary instruments (i.e. currency or endorsed checks) valued more than $10,000 on a "Report of International Transportation of Currency or Monetary Instruments" FinCEN Form 105. The requirement to report currency on a FinCEN 105 does not apply to imports of gold bullion. You can obtain the form at www.fincen.gov or request one from the CBP Officer if required.
Failure to declare currency in amounts more than $10,000 can result in its seizure.
From Form 105
Travelers carrying currency or other monetary instruments with them shall file FinCEN Form 105 at the time of entry into the United States or at the time of departure from the United States with the Customs officer in charge at any Customs port of entry or departure.
and
PENALTIES: Civil and criminal penalties, including under certain circumstances a fine of not more than $500,000 and Imprisonment of not more than ten years, are provided for failure to file a report, filing a report containing a material omission or misstatement, or filing a false or fraudulent report. In addition, the currency or monetary instrument may be subject to seizure and forfeiture. See 31 U.S.C.5321 and 31 CFR 1010.820; 31 U.S.C. 5322 and 31 CFR 1010.840; 31 U.S.C. 5317 and 31 CFR 1010.830, and U.S.C. 5332.
NOTE that the form itself contains no fields requiring you to justify where the funds were obtained from. However I would expect that declaring the money would trigger some sort of process that flags you as having carried the money across the border. I also expect that whether or not you are questioned more throughly would depend on a lot of things from who you are to where you are coming from.
NOTE also that simply carrying a large sum of currency in the US puts you at risk of Civil Forfeiture by local police departments. There have been many cases of people carrying legitimately obtained funds being stopped by police (for example traffic stop) and having those funds confiscated. You do not even have to be convicted of a crime for this to happen. As such I don't recommend traveling with large sums of cash in the US.
To add onto Peter M's answer (where Civil Asset Forfeiture is mentioned in passing), it should be noted that you have far fewer rights with CBP than you would with police. An Albanian man attempted to fly with $58,100 that wound up seized.
It all began last October, when CBP agents at Cleveland Hopkins International Airport seized $58,100 from Rustem Kazazi, a U.S. citizen, while he was headed to a layover in Newark during a trip back to his native Albania. Rustem, a former Albanian police officer, had worked hard to save up the money with help from his wife, Lejla, and son, Erald, over a dozen years after the family immigrated to Ohio in 2005. Before Rustem could board the plane, CBP agents strip-searched him, interrogated him without a translator and then took his family’s entire life savings, even though they never found anything illegal. They have held the family’s savings for more than seven months, despite never charging anybody with a crime.
Oh, and CBP counted less money than he had on him
Compounding insult and injury, the receipt CBP agents gave Rustem at the time did not note the amount of the seized “U.S. Currency.” CBP later claimed to have taken only $57,330—$770 less than he was actually carrying.
Why did they seize the money? They claim he was involved in drug trafficking
“This is to notify you that Homeland Security Investigations (HSI) seized the property described below at Cleveland, OH on October 24, 2017: $57,330 in U.S. Currency,” the notice states. “Enforcement activity indicates that the currency was involved in a smuggling/drug trafficking/money laundering operation.”
They also accused him of a money laundering crime: structuring. That's where you move money in increments designed to avoid Federal reporting requirements.
In a statement, CBP said that “pursuant to an administrative search of Mr. Kazazi and his bags, TSA agents discovered artfully concealed U.S. currency. Mr. Kazazi provided inconsistent statements regarding the currency, had no verifiable source of income and possessed evidence of structuring activity,” that is, making cash withdrawals of less than $10,000 to avoid reporting requirements.
If I were you, I would not travel with that much cash across the border, declared or otherwise. CBP has immense search power.
As a semi pro poker player I travel in and out of the US with cash over 10k frequently over the years, I have never had any issues when declaring the cash.
I have had my papers in order of course where is the money from etc, where in Vegas or AC will I be staying and for how long.
If leaving the US then receipts etc from casinos and so on.
As long as your papers are in order I would say the chance of anything going wrong is minimal.
