«أَبُو» takes a nominative wāw, while «عُمَرُ» takes a nominative ḍammah.
The requirement is to follow the grammatical case, not to display identical endings. Different noun classes may signal the same case differently.
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Same case does not mean same ending
ʿAṭf al-Bayān: Definition, Examples, and Contrast with Badal
«أَبُو» takes a nominative wāw, while «عُمَرُ» takes a nominative ḍammah.
The requirement is to follow the grammatical case, not to display identical endings. Different noun classes may signal the same case differently.