Is it permissible to use the son's dowry to pay the daughter's dowry due to the parents' inability?

The basic principle is that the dowry is from the husband to the wife, not from the wife to the husband, based on what is stated in the Prophetic Sunnah, and because it falls under the concept of a man's guardianship over a woman and his financial support for her. Furthermore, the dowry is a right for the woman in exchange for the man's enjoyment of her. Therefore, it is not permissible to take money from the son to pay it as a dowry for the daughter.