How can one deal with an antique piece stolen from an antique dealer, given the inability to return it to him or hand it over to the security authorities?

If the artifact is permissible to own, then you must repent and return the stolen item to its owner, even discreetly. If you are certain that this is not its owner, then you must return it to its original owner or give charity on his behalf. However, if it is forbidden, such as an idolized statue, then it has no religious value, and it is preferable to destroy its head and return it to its owner. You are not liable if the destruction was necessary to remove the sin. It must be noted that it is not permissible to take the artifact in the first place, nor to trade in artifacts if it involves harm and imprisonment.