What is the legal basis for mandating the full value when only a part is mutilated?

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The justification for mandating the full value despite partial damage is that the damage is only to a part, so the compensation should not require the loss of ownership over the entire entity. This is analogous to cutting off the penis of a Mudabbir (a slave promised freedom upon owner's death) or cutting off one of a slave's hands or ears. The guarantee is in compensation for what was destroyed, not compensation for the entire entity, thus negating the argument that ownership must transfer due to the compensation for the whole.