Masruq (may Allah have mercy on him) said: I asked Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) about al-suht (unlawful gain): is it bribery in judgement? He said: "No. Whoever does not judge by what Allah has revealed, they are the disbelievers, the wrongdoers and the defiantly disobedient [Surah al-Ma'idah: 44, 45, 47]; rather, al-suht is that a man seeks your help against an injustice, then he gives you a gift and you accept it; that is suht."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20481