What is the legal obligation when a debtor states, 'Katha Dirham' (such and such dirham, genitive)?

Primary text

If the debtor states, 'Katha dirham' (with 'dirham' in the genitive case), he is obligated to pay one hundred dirhams, as this is the minimum number that can be associated with the singular noun in the genitive case (implying addition to one). This is according to the view of Muhammad bin Al-Hasan.