I was talking about flying out of the US and going thru airport security with money. There is no "requirement" for a receipt but its to your advantage to have one. If TSA finds what they believe it over 10K USD, while it is not illegal to fly with large amounts of cash, they must notify local law enforcement and if the airport is large enough they will have DEA on site. They will most likely interview you about the money.....where you got it and what you are going to do with it. If they think your story doesn't cut it they have the authority to confiscate the money. You can get it back but its a pain in the butt. if you have a bank receipt it shows the money originated legitimately. usually customs ( here in the us anyways ) doesn't search a person unless they declare something or they have suspicion.