Johnson seems here to miss Pope's meaning. He could not have meant that he wanted benefits from Halifax before he began to thank him; or certainly he would not have said so even if such had been his real feeling. He is only in a delicate way saying that he wants nothing from Halifax at all; which he conveys by hinting that Halifax has already done something for him (as in granting him his acquaintance, and correcting the passage in question), so that if he is to write on their relationships, it will be to thank him, not to ask for more at his hands.