The Rambam and Tur define the only legitimate form of compensation for a דיין: clearly identifiable compensation for lost work time (שכר בטלה), while all other forms of payment invalidate the judgment.

When and how a דיין may be compensated for his time: the distinction between prohibited payment for judgment (שכר דין) and permitted compensation for lost work (שכר בטלה), as established in the Gemara.

The ideal model for compensating judges (דיינים) according to the Tur and Shulchan Aruch: a community-funded system that preserves independence and avoids any suspicion of bribery or bias.

A gift given to a dayan before the claim is made.

Bribe to a non-Jewish judge and bribes in the form of favours.

Continuing - Does the bribe need to be returned to the giver.

Does the bribe need to be returned to the giver.

Shulchan Aruch on the prohibition of taking a bribe

Rambam on the prohibition of taking a bribe

Continuing in Gemara Ketuvot 105 on the prohibition of taking a bribe