What is the ruling if a dower (mahr) stipulated as two slaves results in one being found to be free or usurped property?

Primary text

If the stipulated dower consists of two slaves, and one is discovered to be free or usurped property, the marriage contract remains valid regarding the portion he legally possesses, and the wife is entitled to the monetary value of the other slave. This ruling is explicitly stated by Ahmad. The entitlement to the value of the compromised portion arises because the stipulated dower cannot be delivered in full as specified, necessitating a substitute valuation for the affected part.