What should one do who took money from a forum owner on condition of remaining in it, then left it, and is now unable to return the money to him because the forum is closed?

This conditional gift is not owned until its condition is fulfilled. If the condition is not met, then it is impermissible to take it, for the Prophet, peace and blessings be upon him, said: "Muslims are bound by their conditions, concerning that which is permissible." Therefore, since you left the forum, you must return the money to its owner. If it is impossible to return it to him or to his heirs, then give it in charity on his behalf. If you find him after having given it in charity, then give him the choice between the charity and taking the money, and the reward for the charity will be yours.