When the father specifies a spouse and the son specifies another, and their dowries differ.
General Chapter
Al-Mughni
Book of Maintenance (Nafaqāt)
Primary text
If the father and son disagree on the specific woman but their respective dowries are equal, the father's choice is prioritized because the marriage pertains to him and the expense is identical. If the dowries differ, the son is not required to pay the greater amount, as he is only bound to provide the minimum amount required for sufficiency. However, the son cannot marry the father to a woman who is ugly or old such that sexual gratification is unattainable.