Sayyiduna Abdullah al-Muzani (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) forbade breaking the coinage current among the Muslims, except that a dirham be broken to obtain silver or a dinar be broken to obtain gold.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11173