Do specific currency units (coins/notes) become determined upon contracting in a sale of money?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
Currency units, like dirhams and dinars, become determined by specification in a monetary contract, meaning ownership is established upon the contract for what they specified. Such specified currency cannot be substituted. If the specified currency turns out to be usurped property, the contract is voided. This is the view held by Malik and Al-Shafi'i.
Supporting text
A contrasting view, attributed to Ahmad and being the position of Abu Hanifa, maintains that currency does not become determined by the contract, thus substitution is permissible, and the contract is not voided if the currency is found to be usurped. Their reasoning is that since currency can be stated generally in a contract without specification, it does not become determined by specification, similar to measures (like the scale or the measure). The prevailing view holds that since it is a countervalue in a contract, it becomes determined by specification, like all other countervalues. Furthermore, it is one of the two countervalues, and thus determined by specification, unlike the items cited as analogy, which are not countervalues but are used only to quantify contracts and define their amount, without ownership being established in them whatsoever.