What is the ruling on money acquired from gambling that the questioner took from his friend, and is it obligatory for him to return the same amount to his friend or to donate it to clear his conscience?

You must repent from watching the evil of gambling, approving of it, and assisting in it. Then, repent from the unlawful money you took, and donate an equivalent amount to the poor and needy, or spend it on charitable causes.

Shaykh al-Islam Ibn Taymiyyah mentioned that a Muslim should avoid usurped money and money acquired through contracts that do not permit its acquisition, if he is aware of it.

Ibn al-Qayyim explained that money received with the giver's consent does not require returning a substitute to the giver. However, it is not permissible for the recipient to consume it; rather, it is impure. The way to dispose of it and complete one's repentance is by donating it as charity.