Is a husband sinful if he stipulates to his second wife that he will not spend the night with her and that her maintenance (nafaqa) will be less than that of the first wife, even with his attempts to please her and visit her during the day, especially since she accepted these conditions?

The condition stipulated by the husband that he will not spend the night with his wife nor financially support her is an invalid condition, but it does not affect the validity of the marriage contract. The wife has the right to demand financial support and the division of nights rightfully due to her according to Islamic law, unless she voluntarily waives her right, as Sawdah (may Allah be pleased with her) did with the Prophet (peace and blessings of Allah be upon him). There is no fault on the husband if what he does is with the mutual consent of both parties; otherwise, he is obligated to provide financial support and the division of nights.