A/N: this is just a little scene I was imagining and decided to share. I hope you are willing to imagine the dancing because I've only written the dialog.
"I believe, Ma'am, I may safely promise you never to dance with him."
It had been over two years since Elizabeth had spoken of never dancing with Mr. Darcy. If anyone had told her, at the time, that she would one day be dancing with him, in his London home, at a ball being held for his sister, she would have been incredulous. Yet she now found herself in just such a situation as she took her place opposite to him in the set.
After a few minutes Elizabeth made some slight observation on the dance. Darcy replied and after a brief silence he added, "This is a very spacious room I believe it is more than adequate for the number of couples."
Elizabeth raised her eyebrows and Mr. Darcy spoke again, "But I suppose it inappropriate, in this case, for me to complement the room as it is my own."
"You might have judged better to have chosen a different topic but I credit you for the effort and give you leave to try again."
Mr. Darcy smiled, "You're very kind… I'm glad so many people where able to attend."
"Your sister must be pleased. I hope she is enjoying her party?"
"I believe she is thank you."
"She looks very pretty tonight," said Elizabeth.
"Yes, I believe her to be the handsomest woman in the room, with the exception of my wife."
"Are you sure you judge properly, Mr. Darcy? Some might suspect you to be blinded by affection."
"My conclusion is based on impartial observation."
"Well, I can agree with you in the case of your sister. Will we hear her play tonight?"
"She may be reluctant to play in front of such a large group," was Mr. Darcy's answer, "but I hope to convince her."
"I wish you every success for I enjoy her playing a great deal."
"I'll tell her you said so. I'm sure the thought of giving you pleasure will overcome her other objections."
"I would not want her to play on my account if it makes her truly unhappy," Elizabeth replied.
"I believe she'll be willing. She isn't as shy as she used to be. My wife has been a good influence on her."
"Mr. Darcy, if you are going to turn every topic into a complement for your wife you will be difficult to converse with."
"Very well, I will choose another topic. I suppose you are still unwilling to discuss books in a ball room?"
"You suppose correctly."
"How are you enjoying your stay in London?"
Elizabeth replied and they continued to speak of trivialities until they had gone down the other dance. After making his bow and thanking her for the pleasure, Darcy asked, "And how do you judge my performance?"
Elizabeth smiled, "You conversed with me splendidly. Now you must ask another young lady to dance."
At the look on Darcy's face Elizabeth added, "You want Georgiana to enjoy her party don't you?"
"And this is to be accomplished by my dancing?"
"It is. If you stand around looking sever people will imagine you to be displeased and it will put a damper on the whole evening."
"If that is the case," Mr. Darcy answered, "I would be better employed in watching you dance. My countenance is always improved by watching you."
"If I dance and you do not I will feel I'm depriving another lady of a partner."
"Then the only logical solution is that we should dance together again."
Elizabeth laughed, "Fitzwilliam! You know very well that you can't be always dancing with your own wife." *
"A convention started, no doubt, by men who are less fortunate than I in their choice of wife."
"Nevertheless, the custom must be obeyed but if you dance four dances with other ladies I will dance with you again."
"Four? You are too cruel!"
Elizabeth's eyes sparkled as she replied, "I will grant you this; two of the ladies may be Miss Bingley and Mrs. Hurst leaving you only two partners with whom you are less acquainted."
Mr. Darcy lowered his voice so that only Elizabeth could hear him, "Dancing with Miss Bingley is hardly a consolation."
Elizabeth was unmoved and Mr. Darcy soon found himself standing up with Miss Bingley.
"I hope your sister is enjoying her party?" asked Miss Bingley, "She must be so pleased to have you throw such a party for her! What an attentive brother you are."
"I believe she is enjoying it, thank you."
"She looks exceptionally well tonight. She has certainly blossomed this last year. I do believe her to be the most handsome woman in the room."
Darcy smiled, "I would like to reply but I've been warned my judgment may be impaired by affection."
"I'm sure your affection for your sister does you credit, Mr. Darcy. Will we hear her play tonight? I do hope we will. I've never heard anyone who I've enjoyed more. "
"I hope to convince her to play."
"Oh I'm sure that if you ask her Mr. Darcy you can have no trouble convincing her… Have I said something to amuse you Mr. Darcy? You looked as if you were about to laugh just then."
"Forgive me; I was reminded of something Mrs. Darcy said earlier."
Miss Bingley's smile only faltered for a moment before she asked, "And how is Mrs. Darcy? I hope she's adjusting to her new life style. But I'm afraid," she continued in a graver tone, "that she may still be practicing some of the habits of economy she picked up at Longbourn."
"If she has habits of economy I hardly consider it a hardship. There are many faults I'd have found more arduous in a wife."
Miss Bingley answered in a tone that she seemed to consider a whisper but actually carried as well her normal voice, "I couldn't help but notice that Mrs. Darcy isn't wearing a new gown tonight but has merely retrimmed one from last year. Of course I never attach much importance to clothing but, as a friend I feel I should warn you, there are those who might think you took advantage of her families' lower circumstances to be stingy in settling on her pin money. If you like I could endeavor to give her a little hint as to what's expected of a woman in her position. "
Mr. Darcy never questioned how Elizabeth spent her allowance. He knew she sent the money she saved to Mrs. Wickham and he didn't begrudge her this kindness. Having no desire to enlighten Miss Bingley on the subject he answered, "If anyone observing my wife in that gown can think only of her allowance I would find him lacking in judgment. I believe she wore it to please me. I'm quite fond of her in it."
Miss Bingley made no reply and after a short pause Mr. Darcy asked her how she was enjoying her stay in London.
When his set with Miss Bingley ended Mr. Darcy went to look for Elizabeth. He had come to realize that he couldn't judge the gown in question objectively unless Elizabeth were to remove it and he hoped to convince her to slip away from their guests long enough to accomplish this. If he failed in that endeavor he might at least convince her to relax her requirements and dance with him again. Surely his experience with Miss Bingley was proof that he shouldn't attempt another conversation without more practice.
*A/N: I'm assuming a married couple doesn't normally dance together based on the way the Elton's behave in Emma. It might not be true but it was a fun thing to have in the story.
Edit: I've since confirmed it would have been considered bad manners for a man to monopolize his wife for the entire ball.