Why did the Prophet (peace be upon him) associate the earnings of a cupper—even though they are lawful—with the dowry of a harlot and the price of a dog—even though they are unlawful—in the Hadith: "The worst of earnings are: the dowry of a harlot, the price of a dog, and the earnings of a cupper"?

The earnings of a cupper (hajjām) are permissible, and the prohibition mentioned in some hadiths is to exalt the profession above the ignobleness of this craft. The Prophet, peace and blessings be upon him, associated it with the dowry of a prostitute and the price of a dog, not to forbid it, but to illustrate the shared baseness and ignobleness between permissible earnings (cupping) and forbidden earnings (the dowry of a prostitute and the price of a dog). This was to encourage the earner to seek lawful and pure earnings that contain no ignobleness. The impurity of the cupper's wage is akin to the impurity of eating garlic and onions; it is an impurity in the earning, not in its essence.