If it is known that the company will borrow with usury (riba), when does one become obliged to sell the shares?

One must verify the nature of the company's activities and its resources before subscribing to shares. It is impermissible to subscribe in a company involved in prohibited activities or dealing with riba (interest/usury). A shareholder is a partner in every transaction conducted by the company, and thus bears the sin of prohibited dealings. Therefore, one must object to such practices and leave the company if it insists on engaging in prohibited transactions. It is permissible for a shareholder to remain in the company until they are certain it will borrow with riba, then they should sell their shares, and declare their objection to riba. As for remaining until the loan is actually taken, that constitutes participation in the prohibited riba-based contract. The more scrupulous approach is to sell the shares before the loan. It becomes obligatory to sell at the time of the actual loan, unless doing so would result in a loss, in which case the shareholder may wait to recover their capital.