“Nobody can think better of Mr. Morland than I do, I am sure. But everybody has their failing, you know, and everybody has a right to do what they like with their own money.” Catherine was hurt by these insinuations. “I am very sure,” said she, “that my father has promised to do as much as he can afford.” (Ch. 16)

I am not sure Isabella thinks the Morlands are less wealthy than she did before. Her comment “everybody has a right to do what they like with their own money” suggests she thinks Mr. Morland is just stingy and won’t provide as much money as he can for his son. Unless there is another reference after this one, I think that is the information she wrote to John about as Mr. Morland might be just as stingy with his daughter.