Receiving payment for giving testimonye
Question

                What is the ruling on a person who pays another who witnessed the truth for his favour, or helped him in a truthful case, in return he gave him an amount of money for witnessing in his favour, knowing that he helped him in what is right and he did not stipulate any fees?.
Praise be to Allah.
    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).