Does a driver's deliberate act of increasing the distance for a passenger—such as extending 2 km to 2.5 km—count as deception and render his earnings unlawful, even if the increase is slight?

It is not permissible for a driver to intentionally take a longer route to increase the fare, because this is deception (ghish). The money earned is not entirely unlawful (haram); rather, only the amount exceeding the usual fare for the customary route is forbidden. This excess amount must be returned to its owner, or, if the owner cannot be reached, it must be given as charity on their behalf.