A thought occurred to me while reading your post. I am not sure I believe this (yet -- still considering it), but what if...?

John Dashwood decided on giving his sisters a certain amount, and spent days telling himself how good a man he is for doing it. Then he tells Fanny, knowing she will be able to talk him out of doing so much.

Perhaps he falls in so readily with his wife's arguments because he knew before going to her that she will afford him the reasoning to justify his inertia. When the ladies leave Norland, he could (quietly, without reference to Fanny) decide, 'well, I can not do neighborly acts for them NOW. And I did promise my father. I had better give them some money, as I originally intended. It is now the only way I may honour my father's request.'