Should he give his zakaah to his son because his salary is low?

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Check below answers in case you are looking for other related questions:
Giving zakaah to someone who wants to get married
Giving zakaah to a poor woman whose husband does not spend on her

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Related Answers:
He wants to get married but he owes a debt to his father; can he take zakaah from him?
Paying off a father’s debt from zakaah
Giving zakaah to one’s mother if her husband does not spend on her
Giving zakaah to someone who wants to get married
Giving zakaah to a poor woman whose husband does not spend on her

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