He lent money to some of his colleagues and bought shares in some companies – how should he pay zakaah?

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Check below answers in case you are looking for other related questions:
Ruling on zakaah on a debt – should it be paid from other wealth?
Is zakaah required of one who is in debt?

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Related Answers:
Detailed discussion of zakaah on shares
Zakaah on stagnant real estate shares
Should he pay zakaah on shares that were given to him by the company and over which he has no control?
Ruling on zakaah on a debt – should it be paid from other wealth?
Is zakaah required of one who is in debt?

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