It's interesting to think about what Henry Dashwood did have in mind for John to do, and why he didn't just spell it out for him. It sounds like they had a long talk.

Ideally, I suppose, he might have wished that John would let Mrs. Dashwood and the girls stay at Norland, or perhaps in a smaller home on or near the property, much as Jane Austen and sister did at Chawton cottage, or as Sir John offered them at Barton. John might also have agreed to be sure each girl would have a suitable dowry--or some kind of dowry beyond 1000 pounds. I suppose that's what his idea of giving them 1000 pounds each was for, in addition to the 50 pounds a year income off it.

It sounds like there was bad blood between Fanny and the family from Day One. We are told she had "never been a favourite" with her husband's family and that Mrs. Dashwood had felt 'contempt' for Fanny since 'very early in their acquaintance.' Henry Dashwood cannot have been unaware of this. He may not have wished to extract a promise for anything so specific.