פירוש על עבודה זרה 104:16

When he was younger, Rabbi thought that since the idol was owned jointly by the Jew and non-Jew, the Jew was worshipping it on account of the sanctity ascribed to it by the non-Jew. When the non-Jew annulled it, the Jew would no longer accord it any sanctity and therefore it is permitted.

When he got older, perhaps more cynical, he believed that the Jew worshipped the idol on his own account. Therefore, the idolater’s annulment would not be effective and the idol would remain prohibited.