Union. Freeing slaves happened in all societies that have slavery.
Romans freed slaves, whenever they want to, but they didn't abolish slavery.
But freeing slaves doesn't actually mean ACTUALLY abolishing slavery altogether.
There is a difference , 'Free the Slaves' in Qur'an is expressed in imperative mode , a direct instruction of Almighty GOD . It is ranked as vital as believing the GOD [Ref. Q2.177] .
Hence it is not just an option rather an obligation .
Another place in Qur'an it is said that when a slave is asking for freedom , the master should accept it and should support him/her economically to ease his/her establishment in the society . [Ref. Q24.33]
Other verses encourage the people to free the slaves every now and then .
On the top of it there is
not a single verse in Qur'an which advocates slavery nor condones it remotely . If you know any bring it and then we will discuss it .
Muhammad may have freed a few slaves that he owned. But Muhammad didn't abolish slavery, which is the owning or selling of slaves.
Muhammad still allowed for an unspecified number of women and children to be sold as slaves, after beheading 600-900 men who didn't convert to Islam, of Banu Qurayza, in 627.
Look Qur'an is the earliest source of Islam and all other sources like Hadith and Sirah were created some hundreds years after the demise of the Prophet Muhammad and his companions . So I don't really care of what is written in those books .
If you can prove your point from Qur'an that just buzzes me and nothing else .