פירוש על קידושין 45:2

Others interpret the story to refer to a minor slave. If a convert dies, his adult slaves go free according to all opinions. According to Abba Shaul, minors do not become free. Thus Mar Zutra did not have to send the slave to do an errand for him. He would have remained a slave in any case, and Mar Zutra could have just taken him as a slave after R. Judah’s death.