Musannaf Ibn Abi Shaybah

Hazrat Zuhri and Qatadah (may Allah be pleased with them) state that if the People of the Book, a slave, and a child are witnesses, and then the People of the Book accept Islam, the slave is freed, and the child reaches adulthood, then it is permissible for them to give testimony. However, if their testimony was rejected in their previous state, then it is not permissible.

Hazrat Zuhri (may Allah have mercy on him) says that if a slave gives testimony while in slavery and his testimony is rejected, then if he is freed, his testimony (in the same matter in which it was previously rejected) is not valid.

Hazrat Hasan (may Allah be pleased with him) says that if a slave and a dhimmi give testimony and their testimony is rejected, then the slave is freed and the dhimmi becomes Muslim, their testimony is valid.

Ibrahim (may Allah have mercy on him) says that if a slave gives testimony and his testimony is rejected, then he is set free, that testimony is not valid. However, Hazrat Hakam (may Allah have mercy on him) says that his testimony is valid.

Hazrat Abu Salamah says that if a slave gives testimony and his testimony is rejected, then he is set free, his testimony is not considered valid.

Hazrat Shurayh (may Allah have mercy on him) says that if a slave gives testimony and his testimony is rejected, then he is set free, his testimony is not valid.

Hazrat Umar (may Allah be pleased with him) regarding the testimony of a Jew, a Christian, and a slave, states that when they gave testimony which they could not establish (i.e., there were two of them), until the slave was freed and the Jew and Christian became Muslim, then their testimony will be valid, so their testimony is correct.