Modern Commentary
On one hand, you should tell your partner the truth. On the other hand, if Kimberly was talking about cutting out ramen to save money, you know she would've insisted that Brisbane not buy her anything. And if she bought him something but he didn't, that creates an imbalance and an obligation.

Like Brisbane, I have no idea how bustier sizes work. Couldn't even begin to guess. A gift card would be the smart thing, so that Kimberly could get something properly sized. Or just get any size and keep the receipt so she could return it for the right one at some point after Christmas. If you're clever, you explain that it's the only size they had during the Christmas sale so your partner isn't upset about you buying one that's too large or too small. - Terrence