Is it considered backbiting to deny the truthfulness of someone's statement and to say that it is not true, if the statement cannot be believed?

There is no sin in not believing someone who tells you something you do not find truthful. However, one should not accuse them of lying without verification. There is no harm in informing people that a statement is false if you know it to be so; in fact, it may become incumbent if believing it leads to harm. What entails a legal ruling does not fall under backbiting, such as mentioning an error for the sake of advising against someone whose imitation or allure is feared.