Both أَحَدَ and عَشَرَ have their fixed final fatḥah in أَحَدَ عَشَرَ.
Being indeclinable means that their endings do not change like those of an ordinary declinable noun when the expression functions as subject, object or after a preposition.
Nevertheless, the whole compound has a changing syntactic position: nominative, accusative or genitive position. Distinguish the fixed outward form from its grammatical role.
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Al-Murakkabāt — Compound Numerals, the Twelve Exception and Declining Baalbek
Both أَحَدَ and عَشَرَ have their fixed final fatḥah in أَحَدَ عَشَرَ.
Being indeclinable means that their endings do not change like those of an ordinary declinable noun when the expression functions as subject, object or after a preposition.
Nevertheless, the whole compound has a changing syntactic position: nominative, accusative or genitive position. Distinguish the fixed outward form from its grammatical role.