Is the testimony of a master (*sayyid*) concerning the waiver of his manumitted slave (*mukatab*) valid?

Primary text

The testimony of a master concerning his manumitted slave's waiver of preemption, or concerning the purchase of something in which the manumitted slave has a right of preemption, is not accepted because the manumitted slave is considered the master's slave, similar to a slave designated for manumission upon the master's death (*mudabbir*). Any benefit accruing to the *mukatab* benefits the master, either by reducing the price he must pay or by simplifying his ability to pay.