If a manumitted slave (*Mukatab*) claims authorization, is the testimony of his master, the master's sons, or the master's parents accepted?

Primary text

If a *Mukatab* claims authorization, the testimony of his master, the master's sons, or the master's parents is not accepted. This is because the master testifies for his slave, the sons testify for their father's slave, and the parents testify for their son's slave.