Why did the Prophet (peace be upon him) replace the slave who pledged allegiance to him with two black slaves? Does that indicate his dislike for black people, or was it because the two slaves were polytheists? And did he own slaves?

The objector needs to prove that the slave whom the Prophet (peace be upon him) bought was white. The flaw in the objector's statement becomes apparent in that the Prophet (peace be upon him) did not differentiate between people except by piety, in accordance with the Almighty's saying: ﴿إِنَّ أَكْرَمَكُمْ عِنْدَ اللَّهِ أَتْقَاكُمْ﴾ [Indeed, the most noble of you in the sight of Allah is the most righteous of you], and his (peace be upon him) saying: “Verily, there is no superiority for an Arab over a non-Arab… except by piety.” The Prophet (peace be upon him) chose Bilal as a muezzin, and Usama ibn Zayd, who was very dark-skinned, was the most beloved of people to him, even being nicknamed the beloved of the Messenger of Allah.

The Hadith about selling a slave was narrated by Muslim, and it states that the Prophet (peace be upon him) bought a slave who had pledged allegiance to emigrate, for two black slaves. This indicates that the slave was owned, and thus it is not permissible for him to pledge allegiance without his master's permission. The Prophet (peace be upon him) bought the slave to honor him and uphold his pledge without infringing upon his master's right. The Prophet bought the slave for two slaves to satisfy the seller and achieve the principle of mutual consent in the sale, or because the slave's value exceeded that of one slave, or to appease the seller, or because the seller was the one who requested this price.