If he knows that the company is going to take a riba-based loan, when should he sell his shares?

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Check below answers in case you are looking for other related questions:
If a doctor does an abortion, does he have to pay diyah (blood money) and offer expiation?
Abortion of physically deformed foetus

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Related Answers:
Interest based loans
Shares in telecommunications and service companies
Paying zakaah ahead of time and how to pay zakaah if his money is in the Islamic bank
If a doctor does an abortion, does he have to pay diyah (blood money) and offer expiation?
Abortion of physically deformed foetus

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