It is not permissible to receive payment for giving testimony, because the one who has testimony to give must give it for the sake of Allaah, and in order to establish truth and dispel injustice. Allaah says (interpretation of the meaning): 

“as witnesses to Allaah”

[al-Nisa’ 4:135] 

“And establish the testimony for Allaah”

[al-Talaaq 65:2] – and not for the sake of worldly gain. 

And Allaah says (interpretation of the meaning):  

“And conceal not the evidence, for he who hides it, surely, his heart is sinful. And Allaah is All-Knower of what you do”

[al-Baqarah 2:283]

The one who has testimony concerning rights must give it without accepting any payment for that, because this is a kind of worship that Allaah has enjoined in the verses (interpretation of the meaning):  

“And establish the testimony for Allaah”

[al-Talaaq 65:2]

“as witnesses to Allaah”

[al-Nisa’ 4:135]

With regard to the one who helps you in your dispute or in a case, if he helped you in the sense that he argued on your behalf, and took on the case as your deputy or representative, then there is no reason why you should not give him some money in return for his efforts. That includes what is done by lawyers, who represent plaintiffs and argue on their behalf. They receive payment for their efforts because they are agents of the people they represent in their cases. But it is not permissible to accept payment for giving testimony under any circumstances. 

The same applies to the judge who passes judgement between people. It is not permissible for him to take any payment for his judgement. If he does accept anything then this is a bribe which is forbidden by Allaah, may He be exalted, and His Messenger (peace and blessings of Allaah be upon him), and the scholars are unanimously agreed that it is haraam. But as we have mentioned, it is permissible for the agent or representative in a dispute to receive payment in return for his efforts, if he stipulated that, or if the one who won the case wants to reward him with something for his efforts. And Allaah knows best. End quote. 

Majmoo’ Fataawa al-Shaykh Saalih al-Fawzaan (1/80).