You bill for the procedures you perform. If the insurance company doesn't agree, they simply won't pay for it. That's why it's so important to settle payment terms with your patients prior to beginning treatment. Absent evidence of insurance fraud, though, they won't spend the time or money necessary to take legal action against you.
Incidentally, if you're replacing that restoration, it shouldn't be with a direct composite. Rebuilding an entire cusp is a lot to ask from a resin. Your patient would be much better off with an indirect restoration in the situation you're describing, if they elect to have the amalgam replaced.