First, I am certain that he completely anticipated her response. Desperate pleas aside, she wouldn't have trusted that he would have really left Harry alive. He gave her a few seconds to think, come to that conclusion, and then she tried to the only option she thought she had left.

Second, the wording was:

"Very well," said the voice of death, now sounding coldly amused, "I accept the bargain. Yourself to die, and the child to live. Now drop your wand so that I can murder you."

The only part of the bargain that she had to uphold was dying.

Though I'm not at all certain that the scene is what it looks like.