I am confused here as to what you and your daughter had agreed upon,
If you and your husband promised her £500 when she found accomodation then I too think you are morally obliged to hand it over now.
If the promise was made on the condition that she saved the £1400 you mention, and she has in reality only saved £800, then you may justifiably feel that you have no obligation to give her the promised £500.
That said, if you can afford to do so, hand over the money you promised her. If you don't you are spoiling any chance of a decent relationship with her in the future. This may actually already be spoiled, as she is obviously cross with you.
She is 23, so how much she earns, uses or has in the bank is, as she recently told you, no concern of yours, unless you have at some earlier point stood guarantor for her. It would have been more graceful of her not to ask you to mind your own business, but that is the sort of remark we sometimes have to let lie if we want a good relationship to our adult children.
I realise you probably feel that you are throwing good money after bad if you give her £500 now, and I am sadly afraid that that is precisely what you will be doing, but do so and promise yourself that you will not promise or give her any more money in the future. But keep that last to yourself. Nothing in the present situation will get better if you succumb (as I probably would) to the temptation to say, " and don't expect any more from us, ever!"