All perfect praise be to Allah the Lord of the Worlds. May Allah`s peace and blessings be upon our Prophet Mohammad and upon all his family and companions.

Saved gold is liable for Zakah once it reaches the due Nisaab (Minimum amount liable for Zakah) and a Hawl (Lunar year) passes over possessing it. The Hawl of this gold is the same as that of the paper money, which reached Nisaab and the gold was bought with. For example, when the Hawl of the paper money is completed in Sha`ban and gold was bought with it-for saving-in Rajab, the Zakah of the gold is due in Sha`ban of the coming year, not Rajab. This is because the Illah (legal cause) which makes paper money liable for Zakah is the same which makes gold and silver liable for Zakah: both have a price.

Resolution No. (22, 6/5) of the Islamic Fiqh Academy states about the Zakah of paper money: "Zakah is due on paper money once it completes the Nisaab when added to gold, silver and tradable items." On this basis, the paper money, the tradable items, the gold and the silver have the same Hawl. Rather, jurists have stated that the Hawl of tradable items is the Hawl of their origin. I.e. the money with which they were bought. Consequently, this applies to paper money as well as the gold and silver bought with it. 

It is stated in [Al-Mawsoo`ah Al-Fiqhiyah, V.23: P.268] : "The value of tradable items is added to that of gold and silver, then the Nissab of each is completed with it. Ibn Qhoddamah says: "There is no disagreement about this." Similarly, the same ruling applies to paper money in circulation." And Allah the Almighty knows best.