Is it permissible to hit a person who has allowed it, regardless of the severity of the blow or the shedding of blood, and whether the hitting is in exchange for money, for sport, for entertainment, or as an agreed-upon punishment?

No one is permitted to sanction harming oneself through beating, even with one's own permission. The body does not belong to the human being to dispose of in a way that harms it; rather, it belongs to Allah Almighty. Scholars such as al-Qarafi and al-Shatibi have affirmed that the rights of the intellect, the body, and the soul are among the rights that Allah Almighty has over His servants, and a servant is not allowed to waive them.