Besides we know that the human body is an amanat (trust) of God to a human being. At the end of his life, a believer should return this amanat as he received it form God. He is not allowed to donate what he does not own.

The hadith you have quoted prohibits mutilating your enemy whom you kill even the ones you kill in the battle field while fighting in the way of God. Organ donation we believe is totally a different thing.

As for the view that it not allowable to an organ because our body is amanat of God which we must return to Him in the form we received, we believe that the conclusion does not follow directly from the premise. Besides, this premise is not a fact based on any express divine statement. It is but view of the scholars. I do not know any Qur’anic verse or the Prophetic hadith which clearly and explicitly prohibits the apparently good work i.e. organ donation. For a thing to be described as prohibited or haram we need explicit and clear divine command. However, those who are convinced of the view you hold are required to follow what they believe to be the truth. We are not allowed to force our opinion on others. Nor can we hold that all others are wrong and our view is absolutely correct and must be accepted.

God knows best.