Is all of the deceivers' money unlawful, or is the prohibition limited to the portion in which they usurped people's rights?

Short Answer:

"Al-Tatfif" (defrauding) is the reduction of people's rights in weighing and measuring, whether by increasing or decreasing. The money acquired by a defrauder (mutaffif) from people without an equivalent exchange is unlawful to the extent of the defrauding. While the origin of his money is lawful, it has become mixed with the right of another. Whoever has committed this sin is advised to offer sincere repentance to Allah.