Is money earned from selling virtual assets in a game unlawful (haram), knowing that the company producing the game prohibits its sale?

It is permissible to rent out the benefits you obtain from the game-producing company under a lease contract, unless there is a condition preventing it. If a condition prohibiting rental exists, it is obligatory to abide by it, and you are not permitted to rent out these benefits to others. As for the money you earned previously, we hope there is no problem in benefiting from it, due to the difficulty of reaching its owners, and given the difference of opinion among scholars regarding the validity of this condition.