«لَا تَبِيعُوا الطَّعَامَ بِالطَّعَامِ إِلَّا سَوَاءً بِسَوَاءٍ»
“Do not exchange food for food except on an equal basis.”
The exception permits the equal-exchange situation within a general prohibition. The juristic disagreement concerns how broadly the prohibited class is understood and which transactions meet the legally relevant criterion of equality.
The Hanafi reasoning evaluates the operative basis and the measure of equivalence when discussing certain exchanges not governed by the same measurable criterion. The Shafii analysis presented here gives the general prohibition a broader reach.
This illustrates that interpreting an exception requires attention to the subject, operative basis and scope of the original text, not merely numerical subtraction.