Is a settlement valid if a witness agrees not to testify about a right that necessitates testimony?

Primary text

A settlement by a witness agreeing not to testify regarding a right that legally requires testimony, such as a civil debt or a right belonging to God that is not waived by doubt (like Zakat), is invalid. Receiving compensation for withholding such testimony is impermissible, akin to accepting payment for drinking wine or abandoning prayer.