What is the legal implication when a debtor states, 'Katha (such and such) Dirham' with the noun 'Dirham' in the nominative case (Raf'a)?

Primary text

If the debtor states, 'Lahu 'alayya katha dirham' (I owe you such and such dirhams) with 'dirham' in the nominative case (Raf'u), he is obligated to pay one dirham. This construction means that 'katha' (such and such) is a substitution for 'dirham', establishing the amount as one dirham.