Sahih al-Bukhari Book 49, Hadith 18
A man amongst us declared that his slave would be freed after his death. The Prophet (ﷺ) called for that
slave and sold him. The slave died the same year.
`Utba bin Abi Waqqas authorized his brother Sa`d bin Abi Waqqas to take the son of the slave-girl of Zam`a into his custody, telling him that the boy was his own (illegal) son. When Allah's Messenger (ﷺ) went (to Mecca) at the time of the Conquest, Sa`d took the son of the slave-girl of Zam`a to Allah's Messenger (ﷺ) and also brought 'Abu bin Zam`a with him and said, "O Allah's Messenger (ﷺ)! This is the son of my brother `Utba who authorized me to take him into my custody." 'Abu bin Zam`a said, "O Allah's Messenger (ﷺ)! He is my brother, the son of Zam`a's slave-girl and he was born on his bed." Allah's Messenger (ﷺ) looked at the son of the slave-girl of Zam`a and noticed much resemblance (to `Utba). Allah's Messenger (ﷺ) said, "It is for you, O 'Abu bin Zam`a as he was born on the bed of your father." Allah's Messenger (ﷺ) then told Sauda bint Zam`a to observe veil in the presence of the boy as he noticed the boy's resemblance to `Utba and Sauda was the wife of the Prophet (ﷺ) .
Allah's Messenger (ﷺ) forbade the selling or donating the Wala' of a freed slave.