An istithna' (exception) excludes something from an initially expressed whole, typically with the word illa.
The mustathna minhu is the original total; the mustathna is the excluded amount; and the remainder is what the effective admission leaves payable.
In “I owe ten dirhams except three,” ten is the original total, three is excluded and seven remains. Keeping these concepts separate prevents errors in the rules about excluding everything or excluding something from a different category.
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Exception: terms and structure
Kitab al-Iqrar: Connected Exceptions, Quantities, and Cross-Category Exceptions
An istithna' (exception) excludes something from an initially expressed whole, typically with the word illa.
The mustathna minhu is the original total; the mustathna is the excluded amount; and the remainder is what the effective admission leaves payable.
In “I owe ten dirhams except three,” ten is the original total, three is excluded and seven remains. Keeping these concepts separate prevents errors in the rules about excluding everything or excluding something from a different category.