A person from the Ansār declared that a slave of his would be free after he dies.

A man from the Ansār promised that a slave that he owned would be free after his own death although the master had no other property. As the Prophet (may Allah's peace and blessings be upon him) was told of this, he considered this deal a waste of resources. So, he did not approve of this act. Meanwhile, he sold the slave for eight hundred dirhams and sent the money to the master.
The priority for the poor master was to take care of himself and his family rather than emancipate his productive slave, and thus the indigent family would survive on begging.
Finally, even though this tradition was applicable at some point in history and has no relevance today, something can be learned from it. Slavery is still practiced in some places like Africa, just as it was practiced since the time of Adam until after the coming of Islam. However, the attitude of Islam toward slavery is to eliminate it.