Cops can confiscate cash from anyone any time. If it includes a hundred dollar bill the Peace Officers can actually prove that it was used in an illegal transaction and that is all that is required to keep the bill and any money associated with it. Volusia County Florida financed for a few years, and maybe still does, much of its government by highway stops of elderly retirees going south in their Caddilacs and Lincolns with their life savings in cash to buy condos in sunny South Florida. I don't think any of them has got his money back. If the old couple does not have the money with them in cash, whatever other cash they have was confiscated. It is all good according to Federal Law.
Actually if the police find a hundred dollar bill in the car they can confiscate the car, too, and jewelry and anything else they find that they can convert into cash. I haven't heard of it going further in Volusia than the cash, though.