I think I can add couple of cents to this dialogue.
To start with it is nor right to expect Prophet Muhammad (PBUH) to ban slavery overnight. Let me offer an example.
All of us can agree that child labor is a horrible thing for countries like Bangladesh, India and Pakistan. HOWEVER if you have to cancel it overnight, it would cause more pain to those children and their families unless it is a slow gradual process.
Islam is a religion of middle path and a practical religion (If you understand it properly). So it was clearly against making free people into slaves. The Qur'an says...
It is not Al-Birr (piety, righteousness, and each and every act of obedience to Allâh, etc.) that you turn your faces towards east and (or) west (in prayers); but Al-Birr is (the quality of) the one who believes in Allâh, the Last Day, the Angels, the Book, the Prophets and gives his wealth, in spite of love for it, to the kinsfolk, to the orphans, and to Al-Masâkin (the poor), and to the wayfarer, and to those who ask, and to set slaves free, performs As-Salât (Iqâmat-as-Salât), and gives the Zakât, and who fulfill their covenant when they make it, and who are As-Sâbirin (the patient ones, etc.) in extreme poverty and ailment (disease) and at the time of fighting (during the battles). Such are the people of the truth and they are AlMuttaqûn (pious).
[ Source: The Noble Qur'ân Al-Baqarah 2:177 ]
and Islam motivated it's followers to get out of the system of slavery.
Allâh will not punish you for what is uninentional in your oaths, but He will punish you for your deliberate oaths; for its expiation (a deliberate oath) feed ten Masâkin (poor persons), on a scale of the average of that with which you feed your own families; or clothe them; or manumit a slave. But whosoever cannot afford (that), then he should fast for three days. That is the expiation for the oaths when you have sworn. And protect your oaths (i.e. do not swear much). Thus Allâh make clear to you His Ayât (proofs, evidences, verses, lessons, signs, revelations, etc.) that you may be grateful.
[ Source: The Noble Qur'ân An-Nur 2:221 ]
It also ordered it's followers to treat slaves fairly.
I saw Abu Dhar Al-Ghifari wearing a cloak, and his slave, too, was wearing a cloak. We asked him about that (i.e. how both were wearing similar cloaks). He replied, "Once I abused a man and he complained of me to the Prophet. The Prophet (peace be upon him) asked me, 'Did you abuse him by slighting his mother?' He added, 'Your slaves are your brethren upon whom Allah has given you authority. So, if one has one's brethren under one's control, one should feed them with the like of what one eats and clothe them with the like of what one wears. You should not overburden them with what they cannot bear, and if you do so, help them (in their hard job).' "
[ Source: Hadith - Sahih Al-Bukhari 3.721, Narrated Al Marur bin Suwaid ]