Here's the way it's
supposed to work, using a variation on my earlier hypothetical.
Acting on a complaint from a neighbor, police officers secure a judicial warrant to search the house of a suspected drug dealer. Inside, they find a large quantity of synthetic opioids that can only legally be sold with a doctor's prescription along with a wad of $100 bills, both in the possession of the alleged dealer. They arrest him and impound the drugs. No problem seizing the drugs: they're presumptive contraband, and therefore evidence that a crime has been committed. The bills are ... well, they're just money. But it's a reasonable inference that they were acquired through the sale of some of the illegal drugs. So the police seize the bills as well because they believe they were acquired through the commission of a crime.
Convicting someone of a crime in the United States requires all but incontrovertible evidence: proof "beyond reasonable doubt." Asset forfeitures are civil actions and the government can prevail as long as it is able to demonstrate "by a preponderance of the evidence" that the assets were the proceeds of a crime. (That's the same standard that any other plaintiff in a U.S. civil lawsuit needs to meet.) However in our hypothetical, since the police seized illegal drugs at the same time they seized the $100 bills, that is enough to establish a
prima facie case that the bills are subject to forfeiture unless the owner of the bills can persuade a fact-finder (a judge or jury) by a preponderance of the evidence that the money actually was legally obtained.
Note that it doesn't matter whether the suspected drug dealer is convicted or acquitted at his criminal trial. The asset forfeiture is an independent civil action against the thing sought to be forfeited.
(I'm using $100 bills in this hypothetical, but any asset is potentially subject to civil forfeiture: for example, real estate or personal property purchased with the proceeds of a presumptive crime.)
Anyway, that's the way it's supposed to work. But there are enough well-documented examples of abuses by law enforcement authorities,
including federal agents, to have generated strong opposition at both the state and federal levels to continuing the practice in its current form. The most egregious instances are those where no criminal charge is ever filed against the owner of the assets subjected to civil forfeiture. One possible reform is to require the return of the seized assets whenever there is no corresponding criminal conviction, or at least when no criminal charge is filed. But absent a constitutional amendment, or a federal court ruling that the current practice is prohibited by the U.S. Constitution, that will require legislation or executive action by both the federal government and every state that currently allows civil forfeiture.