What must one do with counterfeit coins (zuyuf) they possess?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
If one possesses counterfeit dirhams, the prescribed action is to melt them down. It is not permissible to exchange them for gold dinars, fulus, or merchandise, as this risks deceiving a Muslim. The ruling of dislike (kurh) stems from the potential for deceiving Muslims, not an absolute prohibition, provided one interprets the situation as one where deception is avoided. The prohibition established by Umar regarding Treasury waste is understood to be due to the possibility of the buyer mixing it with good dirhams and using it to deceive the ignorant.
Supporting text
There is a narration attributed to Umar permitting the spending of debased dirhams in the general marketplace (al-Baqi') for items like worn-out cloth. However, this narration is interpreted by Ahmad as referring to coins that were entirely rejected or voided (nafiya), rather than those merely debased, thus reconciling it with the rulings of prohibition or dislike based on deception.