No sentence, true or false, can be a lie. A lie, or lying, involves mens rea, a guilty mind - i.e. in the case of a lie, intentionality by a person to deceive.
More specifically a lie is generally defined as any statement (the use of a sentence) that is false, known to be false, and is intended to deceive - where a statement is the declarative use of an indicative sentence. There is nothing conceptually wrong with this definition but is the notion of a lie so clear-cut that it excludes another definition? This, for instance:
(1) I state that p to S
(2) I believe that p is false.
(3) By making this statement, I intend to deceive S in regard to p.
Moral terms, such as 'a lie', are seldom open to essentialist definition. If this variant definition is allowed, then p can be a true sentence by means of (the use of) which I lie to S.