Is zakat obligatory on money paid to reserve a commodity (cement) that was delayed in delivery and sold later?

The contract concluded for purchasing cement is a forward sale (salam) contract. The money paid in it is not subject to zakat (definition) for the buyer because it has left his possession upon delivery to the seller. Zakat becomes obligatory on the seller. If the seller is the state, then there is no zakat at all, because this money is considered public funds. As for the selling price of the cement after its receipt, zakat becomes obligatory on the buyer (who is then the seller) if it reaches the نصاب (minimum threshold) and a Hijri year has passed over it. Its amount is a quarter of a tenth (2.5%), provided that the contract has met its conditions, including specifying the delivery date.