What should I do with the money of a person who ruined my account in a game that was entrusted to him, is it permissible for me to withhold it from him in exchange for the damage, and will I be rewarded for my lost money if I return his money to him?

If your friend violates your permission and causes the loss of your account, he is liable for its value, which is determined by experts if it cannot be recovered. You are not permitted to take anything from the money he entrusted to you, for the Prophet, peace and blessings be upon him, said: "Render the trust to the one who entrusted it to you, and do not betray the one who betrayed you." Your established right against him will not be lost with Allah until the Day of Resurrection if he does not fulfill it or if you do not absolve him of it, for the Prophet, peace and blessings be upon him, said: "Whoever has a grievance against his brother concerning his honor or anything else, let him seek his forgiveness today, before there is neither dinar nor dirham."