Why is the wife not entitled to the full value when the stipulated dower is partially defective, similar to the ruling on 'dispersion of a transaction' (tafriq al-safqah)?

Primary text

The ruling does not allow for nullification of the entire stipulated value and return of the full price because the monetary value (qimah) is considered a substitute that is only resorted to when the original item cannot be delivered. Since the owned portion of the slave is deliverable and was specifically named in the contract without defect, reverting to a substitute price for the whole item is not permissible. In contrast, in the dispersion of a transaction, if the contract for the whole item is nullified, the party reverts to the original price, which is not a direct substitute for the specific item sold but rather a return to the initial capital due to the dissolution of the contract.