As for coinage, it is to be made of pure silver, and adulterated coinage does not fall under this ruling. When the affairs of the Persians deteriorated, their coinage deteriorated as well. Thus, when Islam came, their gold and silver coins were not pure, although they served in transactions in place of pure coinage. Their adulteration was overlooked because it had no effect among them, until Islamic dirhams were minted and the adulterated coins could be distinguished from the pure ones.
Aḥmad said, according to the report of Ḥanbal: “If a man were owed a thousand dirhams by another and the latter gave him these dirhams, would that discharge the debt, when they do not conform to the standard people recognize as sound minting and pure silver? And what if the two disagreed, one saying, ‘You have not paid me,’ and the other, ‘I have paid you’? If they resorted to an oath, would he swear that he had paid him in full, when the debt can only be discharged with the silver that Muslims use in their dealings with one another?”
As for spending adulterated coins, the matter must be examined. If their adulteration is concealed, it is not permissible to spend them; this is the only transmitted ruling. If the defect is apparent, there are two reports. The first likewise prohibits it. In the report of Muḥammad ibn Ibrāhīm, when he was asked about counterfeit coins, he said: “It is not lawful.” He was asked, “But the person sees them and knows what they are.” He replied: “Adulteration is forbidden, even when it is apparent.” He likewise said, according to the report of Jaʿfar ibn Muḥammad: “Do not spend a blackened coin until it has been washed, nor counterfeit or debased coins until their nature has been disclosed beforehand.”
The second report permits it. In the report of al-Athram and Ibrāhīm ibn al-Ḥārith, concerning a man who sells dirhams containing inferior pieces in exchange for a dinar, he said: “He should not do so, for he deceives Muslims with them.” Al-Athram asked him, “Do you not say that they are forbidden?” He replied: “I do not say that they are forbidden. I only disapprove of it because he deceives a Muslim with them.”
He also said, according to the report of Ṣāliḥ, concerning dirhams in Bukhara known as al-Musayyibiyyah, most of which were copper with only a small amount of silver in them: “If it is something they have mutually agreed to use among themselves, like the fulūs (copper coins) that people have accepted as customary, I hope there is no harm in it.”