What must we do so that Allah may forgive us and we have not consumed unlawful wealth, and is it permissible to donate now an amount equal to a lost sum that was found and spent, given that we do not know its owner and are away from our city?

The lost item (Luqatah) must be fully described for a full year at the place where it was found and in locations where its owner is likely to be present. If its owner comes and describes it, it must be handed over to him. It is not permissible to benefit from it before defining it. Scholars have differed on the ruling of defining the lost item after a year has passed without definition, but the more accurate view is that it is obligatory to define it even after a year. Therefore, your husband must repent and guarantee the lost item, and appoint someone to define it in the country where it was found. If its owner comes, it should be given to him; otherwise, he should donate it on behalf of the owner. If the owner comes later and is pleased with the donation, he will have the reward. Otherwise, the lost item must be given to him.