Is the ancient doctrine that an officer of the law ceases to have any authority or office when they hold any financial interest in a legal matter before their court.
“Your honour, do you have any financial or commercial interest in this matter?”
Black’s Law Dictionary
It is a rule of equity that a plaintiff [or a Magistrate, all are equal under the law] must come with “clean hands,” i. e., he must be free from reproach in his conduct. But there is this limitation to the rule: that his conduct can only be excepted to in respect to the subject-matter of his claim ; everything else is immaterial.
Pecuniary Bias
When the adjudicating authority has any monetary or financial interest in the dispute, pecuniary bias arises. Any kind of pecuniary interest in the dispute will invalidate the proceedings and disqualify the person acting as a judge.
Source: https://lexlife.in/2020/05/18/principle-of-natural-justice-rule-against-bias/