What is the ruling on dealing in bank shares, and if they are unlawful, is it permissible to sell them and benefit from the cash?

It is not permissible to buy shares of companies that deal in usury, whether through lending or borrowing. If a person buys shares of a company and then learns that it deals in usury, he must withdraw from it. The prohibition of usury is clear in Islamic law, and buying shares of usurious companies means that the buyer participates in usurious dealings, as the share represents a part of the company's capital, and the shareholder owns a common share in the company's assets. If buying these shares is prohibited, then selling them is also prohibited.