the second year arrives, zakah becomes due on it, because of the difference, as the author stated.

Two issues:

The first: Paying the monetary value in zakah.

Most scholars hold that it is not permissible to pay the monetary value except in cases specified by the Sacred Law, such as making up the deficiency in the zakah of camels: “two sheep or twenty dirhams.” The correct view, however, is that it is permissible when there is a benefit or need, whether concerning grazing livestock or produce of the land.

The second: Zakah on shares.

If one buys and sells them, they are subject to the rulings of trade goods: he assesses their value at the completion of the zakah year and pays zakah on them. But if he holds shares seeking profit and growth, then zakah is due on the cash; as for equipment and what is connected to it, no zakah is due on it.