Relying on Scrooge's promise, Sally hired the best specialists and contracted for the best possible physical therapy. She would never have opted for such expensive care in the absence of Scrooge's promise.
What about "forebearance"? Sally acted on Scrooge's promise, but what would it be to "forebear"? To answer, imagine Sally had an offer from a rich Uncle to help pay for her care. She turned the offer down because of Scrooge's promise; that is, she "forbore" (did without) the Uncle's money.
In general, not doing something because of a promise can be just as important as doing something. This is why the promissory estoppel doctrine talks about both "action or forebearance."