Well it would be very easy to figure out.
If their tax return for the past 10 years averages 100k a year...where did the 6.7 Million come from?
Of course.
I think you're missing the point. I'm not saying they're not guilty of some other crime. They might well be, and that is for ordinary detective work to discover. But the presumption should be that they are innocent - until there are good prima facie grounds for suspecting the money is ill-gotten.
Merely being in possession of a large sum of money is not in itself a sufficient grounds for suspecting a crime, at least not to the point of arresting them.
I see no moral difference between carrying cash between two office buildings in the same city or between two countries. It is not a crime to fail to declare cash when moving from Rondebosch to Rosebank, and neither should it be when taking cash from Pretoria to Paris or anywhere else on the planet.
I would be outraged if you were arrested for failing to declare the cash in your possession when travelling from Benoni to Berario. The same applies when travelling internationally.
It is your money, not the State's or anyone else's. Until proven otherwise.
If the cops suspect you of committing a crime simply because you have a large amount of cash then they need to investigate further without just arresting you. There needs to be real grounds for suspecting another crime.
In my view, criminalising the failure to declare the cash is an unjust law because it is not the business of government to say how much money you may possess or move to wherever you wish.
There are no sound financial, economic or moral grounds for requiring citizens to declare to the State what cash they have in their possession when they travel. Of course the State can (and does) quite legitimately require citizens to declare their income for income tax assessment purposes, but that is something done once or twice a year.