פירוש על קידושין 44:14

An animal can be acquired by calling it because an animal moves when its owner wills it. It has far less of an independent will than a human. A slave on the other hand is a human being and moves on his own will. Therefore, the slave cannot be acquired by a purchaser simply calling it.

A minor slave will move according to his master’s will. Therefore, he can be acquired by the purchaser calling him to him.