Fanny is very shrewd in working her husband down to giving nothing.

Though the situation is not funny some of the things she said struck me as funny.

"He did not know what he was talking of, I dare say; ten to one but he was light-headed at the time"

"But then if Mrs. Dashwood should live fifteen years, we shall be completely taken in."

".... people always live for ever when there is any annuity to be paid them; and she is very stout and healthy, and hardly forty"

and then she talks about her very rich mother being tied to to paying "disagreeable" annuities, her "unkind" father in requiring it in his will. Now we know where Fanny gets it from. Is it any wonder he put it in his will after a lifetime spent with her mother?

"They will live so cheap!" ... thanks to you witchy-poo

And of course the way she puffs him up with
"But you have such a generous spirit!"

I'm reminded of the Mr. Knightley quote
"Vanity working on a weak head, produces every sort of mischief."
Maybe John doesn't have a "weak" head per se, but Fanny being the stronger of the two can manipulate him.

But I see John Dashwood as quite the villain because HE was the one who made the promise to his father, and they were his own sisters.

And he says the worst thing in the whole conversation
"Fifteen years! my dear Fanny; her life cannot be worth half that purchase."