If a cop has the authority to search your car, he doesn’t ask permission. Henderson and Boatwrights’ first mistake was consenting to a search. Their second was signing over the cash under ridiculous threat of felony prosecution. An attorney might cost more than the $6,037 but they’d have made it back in spades in the ensuing lawsuit. (Also, it would have become a case of criminal forfeiture, which as the article notes is subject to normal constitutional protections.)
I’m not blaming them; their actions are perfectly understandable in the panic of the moment and unawareness of these systematic abuses, but for future reference: If a cop asks you for permission to do something, just say no.