Is it permissible to accept a gift from a person who was pleased with the completion of his transaction, knowing that the recipient of the gift had no role in completing the transaction, but was merely an informant of its procedures?

There is no harm in accepting this gift, because it is neither a bribe nor among the prohibited gifts to workers. Rather, it is akin to the gift given by one who receives good news to the one who conveyed it. Indeed, Ka'b ibn Malik, may Allah be pleased with him, did this when he took off his two garments and gifted them to the one who informed him of Allah's acceptance of his repentance.