According to the classical Indian school of logic, which fallacy is committed in the statement Fire is eternal because it is produced?

The reason proves the very contrary of the thesis, which is Viruddha.

The thesis claims fire is eternal, offering because it is produced.

Whatever is produced is necessarily non-eternal.

So the mark actually establishes non-eternality, the opposite of the eternality claimed.

A hetu proving the contradictory of the sadhya is the Viruddha hetvabhasa.