Is money earned from designing dresses that reveal the shoulders, half of the leg, or the arms at women's weddings considered unlawful gain?

There is no objection to manufacturing dresses that reveal the arms and mid-calves, because the awrah (intimate parts) of a woman in front of another woman is from the navel to the knee, according to the majority of scholars. Even according to those who say her awrah is in front of her mahrams (unmarriageable kin), revealing the arms and mid-calves is permissible. As for the shoulders, they are not usually exposed. If you wish to refrain from manufacturing dresses that reveal them, you may do so. Nevertheless, the majority of scholars permit it.