Is it permissible for someone who stole unknown sums of money from non-Muslims, and does not possess the means to return them, to intend to donate them [as charity] should he acquire the money, in order to repent from this sin?

A person being a disbeliever does not permit taking their money without right. Therefore, you must repent and return these funds to their owners. If you do not have money, it becomes a debt upon you. For those whose rightful claims you know, you should return their rights to them. For those you do not know, you should return it to their heirs if they exist; otherwise, you should place it in the public welfare of Muslims.