Is it permissible for the orphan’s mother to accept additional sponsorship for him, when there is already a sponsor who covers his needs, and who has instructed her not to accept other sponsorships?

The man's request to the orphan's mother that she not accept sponsorship for her child from anyone else is not binding, and it is not forbidden for her to accept it. However, she does not have the right to combine the sponsorship of this man with the sponsorship of another without the knowledge of each of them, because the fundamental principle for a donor is to intend to give to the needy orphan. So, if she wishes to accept something from someone else, she must inform the donor that the orphan has someone who sponsors him, and she must inform the sponsor of this. If everyone agrees, then there is no harm.