the dress cost was not in your budget when you let her borrow your D as a flower girl, and she can have several options:
1. return the drss and find another flower girl;
2. return the dress, and you will buy a dress for your D at an affordable price, and your girl will still be her flower girl;
3. keep the dress, you will let your D be as her flower girl, but she has to pay for the dress.
You should decide where and how to spend YOUR money. Nothing personal.