Is money earned from a casino considered unlawful (haram) if the original amount (capital) was lawful (halal)?

You have committed a great sin by gambling, and you must offer sincere repentance. The money won from gambling is forbidden ill-gotten gain (suḥt), which must be returned to its owner if known, or given in charity if unknown. Allah Almighty says: "O you who have believed, indeed, intoxicants, gambling, [sacrificing on] stone altars [to idols], and divining arrows are but defilement from the work of Satan, so avoid it that you may be successful." What a gambler earns is impure and unlawful money, and one must clear their conscience by returning it to its owners or to the poor. As for the money you lost, it is permissible to reclaim it from those with whom you gambled, if possible.