When is it impermissible for the lender (Mu'ir) to revoke a loan for use (A'riyah)?

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Revocation of a loan for use is prohibited if its immediate cancellation would cause harm to the borrower (Musta'ir). This is because causing harm to another is forbidden. An example is lending an item, such as a plank for repairing a ship, which the borrower uses to sail into the sea; revocation is forbidden while the ship is at sea. Revocation is permitted before the ship enters the sea or after it exits, as no harm results then.