A diptote is still muʿrab, not mabnī. Its normal genitive marker is fatḥa rather than kasra in the ordinary indefinite/construct-free form.
جَاءَ أَحْمَدُ: nominative with ḍamma.
رَأَيْتُ أَحْمَدَ: accusative with fatḥa.
مَرَرْتُ بِأَحْمَدَ: genitive with fatḥa.
The syntactic case is still genitive; only its visible marker differs.
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Definiteness and Proper-Name Status in Diptote Rules
A diptote is still muʿrab, not mabnī. Its normal genitive marker is fatḥa rather than kasra in the ordinary indefinite/construct-free form.
جَاءَ أَحْمَدُ: nominative with ḍamma.
رَأَيْتُ أَحْمَدَ: accusative with fatḥa.
مَرَرْتُ بِأَحْمَدَ: genitive with fatḥa.
The syntactic case is still genitive; only its visible marker differs.