I don’t think civil forfeiture has anything to do with it. The driver handed the cop $3 voluntarily (and at significant personal expense, it turns out!), I don’t think he necessarily has any legal right to get the money back (though it’s just rude not to return it), and the cop doesn’t need to invoke civil forfeiture to avoid returning it (though I’m not a lawyer).
On the other hand: did the cop trouser the $3, or did he enter it into evidence or otherwise turn it over to the department? While as a citizen he’s not obligated to return money someone hands him, as a cop he’s probably subject to regulations that prohibit accepting a gratuity from the public. It might be entertaining to make trouble for him on such grounds, if you were for example holding a $185-sized grudge against him.
