Commentary for Niddah 6:4

כל הראוי לדון דיני נפשות – all the more so is appropriate/fit to judge monetary matters.

Anyone who is fit to try capital cases is also fit to try monetary cases. But there are those who are fit to try monetary suits and and unfit to try capital cases. The laws regarding who may adjudicate a capital case are more stringent than those for monetary cases. For instance, a convert or a mamzer cannot judge capital cases (see Sanhedrin 4:2).

ויש שראוי לדון דימי ממונות – as for example, a Mamzer, who is fit/kosher to judge monetary matters but is invalid to judge capital crimes (see also, Tractate Sanhedrin, Chapter 4, Mishnah 2 and the Bartenura commentary on the concluding clause of that Mishnah).

Anyone who is eligible to act as judge is eligible to serve as a witness. But there may be one who is eligible to act as witness and not as judge. Similarly, the laws regarding who can serve as a judge are stricter than those for a witness. For instance, an unlearned person can serve as witness, but not as a judge.

ויש שכשר להעיד ואינו כשר לדון – as for example, a person who is blind in one of his eyes. But our - Mishnah is according to Rabbi Meir who expounds on Scripture, as it is written (Deuteronomy 21:5): “and every lawsuit and case of assault is subject to their ruling/ועל-פיהם יהיה כל-ריב וכל-נגע. He makes an analogy between ריב/lawsuit and נגע/case of assault – that just as a case of assault is not with a blind person, as it is written (Leviticus 13:12): “whatever the priest can see/לכל-מראה א=עיני הכהן,” even a lawsuit is not with a blind person. But the Halakha is not like this anonymously taught [Mishnah], for the Sages dispute on that of Rabbi Meir and state that just as we find that the closing of legal proceedings is at night, even though we can’t see נגעים/people afflicted by plagues/suspected leprosy at night, that is so that a blind person in one of his eyes is fit to judge even though he is not kosher/fit to see plagues/suspected leprosy.