MEXICO CITY, Dec 30 (Reuters) - Mexican Finance Minister Arturo Herrera on Wednesday criticized a divisive bill proposed by the country’s ruling party that would force the central bank to buy foreign cash commercial banks are unable to return to the financial system. “This change would only transfer the problem commercial banks have to the central bank because the central bank would have the same problem: how to export dollars,” Herrera told Reuters in an interview in the finance ministry offices. (Reporting by Stefanie Eschenbacher and Anthony Esposito; editing by Diane Craft)