Compose a well-substantiated argument about Austen’s Pride and Prejudice in terms of the problems presented by marriage and agency in an acquisitive society and Austen’s Persuasion as embodying a new sense of the evolution of society and of marriage and agency. Substantiate the answer with apt references to the texts.

I am not sure what is meant by "agency" in this context, but I will assume it means a woman's sense of empowerment in making her own choices about marriage. 
Both these novels are complex, but Pride and Prejudice can be understood as a debate on the reasons for marrying. Charlotte Lucas stands for complete pragmatism in marriage: she accepts Mr. Collins, an awkward man (he steps on Elizabeth's toes while dancing) who the Bennet sisters (and father) laugh at as a buffoon. Mr. Collins lacks social skills, flatters and kisses up to people in power in a shameless way, and turns cruelly on the Bennets when Lydia gets in trouble. Charlotte cannot know what Mr. Collins will do in the case of Lydia when she agrees to marry him, but she knows full well he is a buffoon. She marries him to have an "establishment": he offers her a good, secure income, the assurance that he will inherit the Bennet family estate, and, all importantly, the status in society that comes with being a wife. Charlotte, age 27, fears being the dependent sister of brothers who might resent supporting her. For her, the house, the income, the security, and the status are all far more important than the personal appearance or characteristics of the man she marries. To her, marriage is a financial and status transaction, and she reasons that she has as much chance of happiness in it as anyone else.
Elizabeth, in contrast, represents, at least in her own mind, the idea of marrying for love. She avows very decidedly that she could never marry a man like Mr. Collins. She says could only marry a man she esteems. 
Here is where we have to stay focused on Austen's actual text and not the hunky movie Darcys that fill the movie Elizabeths with repressed sexual passion. In the text, Elizabeth is not in love with Darcy; he piques her interest when he insults her, and she uses her wit to get back at him. This unwittingly ensnares him, but she does not love him. She also does not like him until near the end of the novel. After all, he makes her utterly furious when he proposes to her like an arrogant jerk (or, as my students call him, a buzzkill). Darcy says he is doing her a great favor by marrying her given how important he is, how little money she has, and what idiots her family members are. He really is no charmer.  She softens toward him when she gets a letter explaining his version of the Wickham story, but she still does not love him. It is only while she is touring Pemberley, his estate, that the thought crosses her mind that it could be something to be the mistress of such an extraordinary place. We must note that this is a very acquisitive thought. Does she fall in love with Pemberley or Darcy? In any case, we know that Darcy does rise in her esteem when he bails outs Lydia (again, money talks), and they do achieve a marriage based on mutual respect. However, it certainly seems as if money comes first in the marriage game. One of Austen's ironies in the novel is that Elizabeth, in the end, is not that much different from Charlotte: both achieve a increase in money and status by marrying socially awkward men.
In Persuasion, money again initially rules the roost. On the advice of her mentor, Lady Russell, Anne turns down Wentworth's marriage proposal because his financial prospects are so uncertain. However, we see growth in Austen's outlook. Anne comes to regret that decision and to feel she should have trusted her own heart over the well-meaning pragmatism of her friend. She suffers for turning down a marriage offer for purely financial reasons. 
When Wentworth comes back, he is wealthy, but Austen makes it clear, especially from Anne's body language, that she is love with the man, not his money. He is not the route to some equivalent of Pemberley. In the end, Anne loves and marries him for himself. This is an evolution over Pride and Prejudice.