How is a man's contribution of thirty thousand Riyals to a piece of land belonging to a real estate company zakat-eligible after many years have passed?

This contribution in land trade offerings must be زكاة-purified every year by appraising it at its current price. If the value of the shares was thirty thousand and became sixty thousand at the end of the year, then zakat must be paid on the sixty thousand. If it became ten thousand, then zakat must be paid on the ten thousand. The zakat for each past year is to be paid, and even if the shares have not yet been sold, they are to be زكاة-purified upon their sale.