As a single working mom raising two kids by myself, I knew better than to "promise" my kids anything.
I make pretty good money when I'm healthy, but I've been off work with a broken leg, I've been hospitalized with shingles and pneumonia. When I'm not working, obviously, money is not abundant. My kids have NEVER gone without the necessities, EVER. I never paid them for "chores" and I never paid them for good grades. That's not to say they weren't rewarded, because they were. But, I never would have promised them something expensive if there was any hint of a chance that I couldn't follow through with it. As a result, guess what.........I never had kids that demanded I follow through.
When I was married, I had kind of the same issue you are facing. My husband constantly was spending tons of money on his son from his first marriage and giving him things because he "promised" him. Well, I have to admit, I thought his argument was pretty ridiculous when it came to, "I promised him when he was 9 years old that I would buy him a car at 16. I REFUSE to break my promise!" At 16, the kid was on the verge of being kicked out of school, he was truant, cut school constantly, and my husband apparently believed it would get better if the kid had wheels. WRONG. His attendance got even worse when he actually had a way to drive around.
All my husband focused on was not breaking a promise.
In your case, the girl followed through on her end. I believe she should be rewarded, but not to the detriment of the rest of the family during difficult times and not just because your husband can't break a "promise" he made.
My point is about promises.
I promise to always love you, I promise to always be here, I promise to be proud of you no matter what.
Monetary promises can be a slippery slope, and I truly believe that well-adjusted kids understand the concept of not being able to be given everything they want. They can wish for something, they can want for something. No problem there. However, it doesn't hurt kids to understand that Daddy or Mommy lost their job or aren't making the same money temporarily and the main thing is having a house and food and trying to take care of everyone as a team effort is how it has to be for now.
That said, I think you and your husband should talk about how to come up with a compromise. Your daughter might have to wait a bit, but you can very likely find something for far less than $400. My son, who is 17 now, never asks for anything, but he mentioned a techno doo-hickey and I priced them at Radio Shack and other places. The price was out of the question, and I told him so.
We actually found one on e-Bay and it was delivered just yesterday.
Brand new, an iPod accessory he'd been wanting. Including shipping, it was less than $15. I didn't even have to bid. It was a "Buy it Now" thing.
My son had to wait, but he got exactly what he wanted and it cost almost nothing. He didn't get one for his birthday in June, but he got one yesterday.
He's happy.
It's okay for kids to wait. It's okay for them to know they earned something, but they might not get it immediately.
I believe that the concept of instant gratification does more harm than good, but that's just my opinion.
Best wishes.