According to German grammar,
mit should always be with Dativ. By German adjective rule, the sentence “Bezahlt man mit Schwarzer Franken” should be as mit Schwarzen Franken (I guess) because Franken is the noun as m., nt.,pl. but not f.
The German adjective rule by Dativ here, only feminine singular noun will have Schwarzer. Maskulin, Neutrum are with Schwarzem. Plural with Schwarzen. None of them is Schwarzer.
So may I seek help from anyone here to help clear my confusion on this part? Much appreciated for your help in advance.