«وَلَوْ شِئْتُ أَنْ أَبْكِي دَمًا لَبَكَيْتُهُ»
And had I wished to weep blood, I would have wept it.
Naming «دمًا» is purposeful. Ordinary weeping is associated with tears; weeping blood presents a strange and intensified image. A bare “Had I wished, I would have wept” would not convey that extraordinary object.
Hence the object after a verb of willing is not invariably deleted. If it is readily inferred it may be omitted, but if its unexpected quality is the rhetorical point, expressing it can be essential.