Act 1, Scene 1 starts with Mirabell losing a game of cards to Fainall in the chocolate house. Finall uses a metaphor to describe why he doesn't want to play another game:
No, I’ll give you your revenge another time, when you are not so indifferent; you are thinking of something else now, and play too negligently. The coldness of a losing gamester lessens the pleasure of the winner. I’d no more play with a man that slighted his ill fortune, than I’d make love to a woman who undervalued the loss of her reputation.
Fainall doesn't want to play another game while Mirabell is thinking about other things because it is not as fun to win against someone who doesn't care about losing. He compares it to flirting with a woman who doesn't feel like it matters what other people will think. Appearances were a big deal in this time period. Even though a lot of people violated so-called norms regarding virtue and chastity, women could face dire social consequences for failing to at least act like they were virtuous and chaste. Fainall only wants to flirt with a woman who has something (her reputation, and thus her marriage prospects) to lose by flirting with him. Likewise, he only wants to beat Mirabell at cards if Mirabell is in a mood to feel the pain of losing.
The metaphor reveals that Fainall relishes in people's pain and is probably not a very loyal husband. He is the villain of the play, after all. But it also reveals something about the world these characters are living in. It is a world that rewards and punishes people based on appearances and reputations, and it can be a highly competitive world. Although neither character states it outright, the card game is a metaphor not only for their rivalry, but also for their society. Strict social structures and rules, like those that govern a card game, allow people to look like good sports while they tear each other apart. For someone like Fainall, the social game is not even worth playing if someone is not getting hurt.
In Act 3, Scene 10, Millamant and Marwood develop an elaborate simile-turned-metaphor comparing acquaintances to clothing:
MILLAMANT: Well, ’tis a lamentable thing, I’ll swear, that one has not the liberty of choosing one’s acquaintance as one does one’s clothes. [...] I could consent to wear ’em, if they would wear alike, but fools never wear out – they are such drap-du-Berry things, without one could give ’em to one’s chambermaid after a day or two.
Millamant describes getting tired of acquaintances, and she complains that when she tries to "wear" fools like clothes for a little while, it's hard to get rid of them. She seems to decide partway through that she likes this comparison. She picks up steam and begins using a direct metaphor, simply talking about clothes made out of dull woolen cloth (drap-du-Berry) when she means fools. The only difference between fools and this drab clothing, she says, is that you can't give fools to your maid when you are done with them.
This metaphor is a callback to Millamant's earlier claim that she used love poems from suitors to curl her hair. Millamant prefers to think about suitors as fools rather than wits, and she prefers to think about fools as clothing and accessories rather than real people. This is why she was rather cruel to Mirabell when she broke up with him in Act 2: she simply wants to get rid of him. She can't pass him off to a servant because that wouldn't be an advantageous match for him. There is a bit of a sense here that Millamant worries men want her for her money instead of for herself.
Marwood sees through Millamant's seeming disdain for all the men who have shown an interest in her. She accuses her of "putting on" fools like Petulant and Witwoud (who would be easy for her to "take off") to conceal the fact that Mirabell is sticking to her. Mirabell is the only clothing Millamant can't get rid of, and Marwood wants Millamant to admit already that she likes him.
At the end of Act 5, scene 14, Mirabell closes the play by handing Mrs. Fainall (Arabella) the deed to her home so that she will now have control of her fortune (monetary and otherwise) within her marriage. The deed is a metaphor for the independence Millamant wants and that Arabella previously seemed to have lost in marriage:
In the meantime, madam, let me before these witnesses restore to you this deed of trust. It may be a means, well managed, to make you live easily together. From hence let those be warned, who mean to wed,
Lest mutual falsehood stain the bridal bed;
For each deceiver to his cost may find,
That marriage frauds too oft are paid in kind.
The "deed of trust" refers to the document that entrusts an estate to the person who holds the document. There is a play on words, though, because Mirabell is restoring to Arabella the ability to decide who to trust with her wealth and independence. It's important that Mirabell hands Arabella the deed right before his parting wisdom that "marriage frauds too oft are paid in kind." Fraud has two meanings: using marriage to steal money and, as Fainall has done, using marriage to control and abuse someone. Fainall thought he was going to get rich off of Arabella by marrying her. Mirabell pulls the rug out from under him and frames it as comeuppance for Fainall's abuse of Arabella.
Congreve has a complicated stance on marrying for money that he sums up here. Marriage in this period was especially tied to moving money around. Congreve isn't necessarily against marrying for money. If Fainall had married Arabella in part for her money but had treated her well and truly loved her, it would not necessarily have been "marriage fraud." But because Fainall does not love or even respect Arabella, marrying her is fraud. Essentially, he wants to steal her wealth and independence instead of sharing in her wealth and acting as her partner. This is exactly the arrangement Millamant fears.
Mirabell serves as a good contrast for Fainall. He cares about the dowry Millamant will bring with her into her marriage, but he honestly loves her as well. And unlike Fainall, Mirabell gets what he wants at the end of the play. Congreve demonstrates with this ending that people with pure intentions will find a way to win out over "frauds." Even Arabella, who has behaved honorably throughout the play, gets her independence and wealth back from the man who tried to defraud her.