What is the sin and expiation incurred by a young man who assaulted another by beating him until blood flowed from him, and he does not know whether the injured person died or not, and is expiation or blood money incumbent upon him?

The details of the matter must be clarified to determine whether the assailant was oppressive or oppressed, and the nature of the offense. Assaulting a Muslim is forbidden, and one must repent from it. The victim must be sought to ascertain his fate. If he died from the blow and the striking was semi-intentional (with an instrument not usually fatal) or intentional (with an instrument usually fatal), each has its own ruling. If he did not die and the assailant was the aggressor, then he is subject to retaliation (qisas) or a prescribed blood money (diyah); otherwise, it is a matter for a just judicial ruling. If the assailant was oppressed and defended himself, then there is no blame upon him. The defense should be with the lighter option unless the oppressor cannot be repelled except by what repels him. These are general rulings that do not suffice in place of consulting the competent Sharia judiciary.