Praise be to Allah the Lord of the Worlds.
According to Islamic law, Istisna` is a contract whereby a party undertakes to manufacture, build or construct assets, with an obligation from the manufacturer or producer to deliver them to the customer upon completion, and the price is either paid on the spot or deferred. Hanafi jurists have permitted Istisna` contract out of Istihsan (application of discretion in legal matters).About the permissibility of Istisna`, Al-Kasani said: "By analogy to Sharia texts, Istisna` isn`t permissible because the Messenger of Allah forbade selling what one doesn`t have, but he (PBUH) allowed Salam (The price is cash but the supply of the purchased goods is deferred). Moreover, Istisna` is allowed out of Istihsan because there is consensus amongst people on it." {Badai`I Al-Sanaa`I, 5/2}.
According to the preponderant opinion of the late Hanafie jurists, Istisna` is a binding contract.
Goods purchased under an Istisna` contract are a debt due from the manufacturer (seller) who is required to deliver them within the time period specified in the contract and their Zakat is similar to that of debts. If the goods are set for trade, then the purchaser should pay their Zakat after one full lunar year elapses and according to the price that he had paid to the manufacturer. However, if the goods aren`t for resale, then they aren`t liable for Zakat. Sharia standards of AAOIFI state: "The price of the Goods, which the company has purchased under an Istisna` contract for resale, should be added to other assets liable for Zakat."
In conclusion, Zakat of goods bought for resale is due on the purchaser where he should pay it after one full lunar year elapses and in accordance with the price that he had paid to the manufacturer. As for the amount which he had paid, it got out of his possession, so it isn`t liable for Zakat. And Allah the almighty knows best.