This is not how the I-864 works.
Please read the instructions to understand what you signed:
You are not obligated to keep her at a certain income level. You are, however, obligated to support her if it becomes necessary. If you think she may end up having to rely on public benefits, then you will want to support her financially to prevent that from happening. But keep in mind, the I-864 is a contract between you and the Government, not between you and her.
It won't hurt to discuss your situation with an attorney and see what they recommend, but I definitely would not just pay her what she is asking for.