What is the legal concept of lineage as a condition for compatibility (kafa'ah) in marriage? Is compatibility in lineage required for a man of noble birth? And does Islamic law find fault with a man marrying a woman of lower lineage than his own?

Compatibility in marriage is considered from the husband's side, not the wife's. The husband must be compatible with the woman and her equal, but the reverse is not a condition. As for a noble husband, it is not considered a defect when he marries someone of a lower status than him. The Shari'ah has commanded husbands to choose women of religious commitment. Allah Almighty says: "And a believing slave woman is better than a polytheist, even though she might please you." [Al-Baqarah: 221]. In the Hadith: "So choose the one who is religious, and may your hands be soiled (with dust - i.e., may you prosper)." Islam does not fault a man for marrying a woman of lower lineage, as long as she is righteous. The questioner's obedience to his mother in choosing a wife is obligatory, as long as it does not lead him to disobey Allah concerning the woman he leaves for his mother's sake, because obedience to Allah takes precedence.