Reference for Ketubot 209:11

הני מילי היכא דשקיל בתורת שוחד קרנא בתורת אגרא הוה שקיל ובתורת אגרא מי שרי והתנן הנוטל שכר לדון דיניו בטלין הנ"מ אגר דינא קרנא אגר בטילא הוה שקיל

This applies only where [the judge] takes [the gift] as a bribe, but Karna took [the two istira] as a fee. But is it permissible [for a judge to take money] as a fee. Have we not in fact learned: One who takes wages for adjudicating, his judgments are void? This applies only to a fee for pronouncing judgment; Karna took compensation for loss of other work.

Jastrow