In kafā bi-llāhi shahīdan, bāʾ is commonly parsed as additional; Allāhi is formally genitive but syntactically nominative as subject; shahīdan is an accusative specification/tamyīz.
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Additional Bāʾ with Kafā
Additional Particles: Seven Forms, Emphasis and Syntactic Case
In kafā bi-llāhi shahīdan, bāʾ is commonly parsed as additional; Allāhi is formally genitive but syntactically nominative as subject; shahīdan is an accusative specification/tamyīz.