«وَمَا ضَنَّ إِلَّا غَبِينٌ، وَلَا غَبَنَ إِلَّا ضَنِينٌ»
Only a fool withholds, and only a miser cheats.
Both mā ḍanna illā ghabīnun and lā ghabana illā ḍanīnun restrict the subject through negation and exception. The adjectives for fool and miser are nominative grammatical subjects, not accusative objects.
The sweeping construction gives the maxims rhetorical force. It should be read as evaluative praise and blame, not as a verified statistical law about everyone who spends or withholds.