The significance of the bargaining scene between Mirabell and Millamant in Congreve's play The Way of the Word is actually manifold.
The first important thing is that they are barganing how each of them will allow each other to behave once the marriage is in effect. This is a parody on the social expectations of men and women upon becoming husband and wife. The requirements make the union far from romantic. Instead, the parties involved are expected to comply with formalities that continuously reminds them that their union is one based on networking and convenience, rather than love.
Another important significance is that they both discover each other's penchants with this behavior pact, and wonder about each other. Again, this is satirical. Millamant says that she wants to be free, and allowed basically to do as she pleases. Mirabell takes this sourly and says that his future bride better not be scandalous nor a "fashion victim". In turn, Millamant takes that personal and cannot believe he would think her to be that way. Even more funny is the fact that all this weird transaction has to be rushed as another character, Fainall, enters the scene.
Yet, it is possible that one of the most important parts of the bargaining scene is the underlying shallowness of the pact. Mirabell says that, upon marriage, he would be exalted to the rank of husband. Contrastingly, Millamant says that she will be demoted to the rank of wife. This is a clear indicator that Millamant is not marriage material, and that Mirabell may not be the dream husband that we assume he wants to be. Hence, the significance here is that Mirabell and Millamant are rushing through the very complex process of pre-nuptial planning with very little evidence of what they really want out of their marriage.