I'm not sure which exchange you are referring to...
The exchanges between Benedick and Beatrice in the first half of Shakespeare's Much Ado About Nothing are witty banter (verbal irony: sarcasm, overstatement, and understatement) that show that their two senses of humor make them soul mates. Really, their verbal "battle of the sexes" attacks on each other are decoys to mask the true love that they feel. It is said that if a couple can laugh together, then they can stay together.
Later in the play, Beatrice summons Benedick to act on his words and challenge Claudio for defaming Hero, her cousin. Actions speak louder than words, and Benedick's challenge in defense of a man's slander of a woman is a testament to how far Benedick has overcome his sexism and how much he really loves Beatrice. In this exchange, Benedick effectively marries Beatrice:
BEN: Tarry, good Beatrice. By this hand, I love thee.
BEA: Use it for my love some other way than swearing by it.
BEN: Think you in your soul the Count Claudio hath wronged Hero?
BEA: Yea, as sure as I have a thought or a soul.
BEN: Enough, I am engaged; I will challenge him. I will
kiss your hand, and so I leave you. By this hand,
Claudio shall render me a dear account. As you
hear of me, so think of me. Go, comfort your
cousin: I must say she is dead: and so, farewell.