פירוש על עבודה זרה 83:19

An idolater can annul an idol, but a Jew cannot. So if we do not know who cut the dragon idol’s head off, the idol is permitted. In cases of doubt, the law is lenient. But if we know a Jew cut the head off, the idol remains prohibited. This is essentially the same difficulty against Resh Lakish as we saw above. Why isn’t this considered an annulled idol, like the case of an idol that broke on its own?
Again, the Talmud solves this according to Rava. Technically, the idol is annulled, but we do not permit it lest the Jew come to own the idol, in which case the idol can never be annulled.