Based on the context of the passage, what does “with only seven thousand pounds” (Sentence 1) mean?

This is a passage by a British author, and so recognizing that "pounds" is a unit of currency is key. However, the entire passage concerns the financial constraints and rules governing marriage. Therefore, even if you weren't familiar with the 19th-century dowry system, you could make an educated guess that "with only seven thousand pounds" describes the bride's financial qualifications for marriage: that is, her dowry.