I'm a little uncertain what your question is, but I hope I can address it.

Miss Maria's dowry was seven thousand pounds. As I understand it, this is non-negotiable. When she marries, the seven thousand go with her.

As to the latter part of the quote, I believe her uncle (and all Huntingdon) is expressing amazement that Bertram would be interested in a woman with less than ten thousand pounds. With his rank and wealth, he could do much better. Thus, Miss Maria is three thousand short, but had the exceptional good luck to marry him anyway.