Reference for Ketubot 209:9

וכ"ת ה"מ היכא דלא שקיל מתרוייהו דלמא אתי לאצלויי דינא קרנא כיון דשקיל מתרוייהו לא אתי לאצלויי דינא וכי לא אתי לאצלויי דינא מי שרי

And should you say that this applies only where he does not take from both [litigants] since he might [in consequence] tilt the judgment in favor of one, but Karna, since he took [the same amount] from both parties, would not come to tilt the judgment. But is it really permitted even where one would not come to tilt judgment?

Jastrow