لا تبيعوا الدرهم بالدرهمين، ولا الصاع بالصاعين
Do not exchange one dirham for two dirhams, or one saʿ for two saʿs.
Here saʿ names a measure. In a sale prohibition, however, the legally relevant matter is the measured commodity and the exchange, not trading empty measuring vessels. The expression is therefore understood through its relation to what is measured.
The example shows why the literal vessel and the figurative measured contents should not be combined as two independent intended objects of this one occurrence. The context first identifies the object to which the ruling relates.