Selling gold to be handed over one year later with an increase in the amount if the price drops
Question

                Now Jewelers have a new scheme. Suppose today price for gold per gram is 10$. I pay 50$ to jeweler and i have 5 grams in my account. Scheme is :

After a year if gold per gram raises to 25$, when i withdraw gold, I get same 5 grams of gold.

But instead gold per gram falls to 5$, when i withdraw gold, I get 10 grams of gold for the value of 50$ i paid while booking the gold.

Is this scheme permissible for us?

If yes, how to deal zakath on this ?.
Praise be to Allah.
    It is not permissible to trade in this manner, because the 
    sale of gold and silver is subject to the condition that the exchange be 
    made on the spot, in the same sitting as that in which the transaction is 
    agreed upon. And the condition of a sale being valid is that it should be 
    definitive and not subject to any fluctuation. 

    If a person buys five grams of gold for fifty dollars, he 
    must take possession of the gold immediately and it is not permissible for 
    him to delay taking possession of it until a year or month or day has 
    passed. 

    In cases where it is not stipulated that possession be taken 
    of the sold item immediately, the transaction must be clear and there should 
    be no fluctuation or ambiguity concerning it. Sharee‘ah permits the salam 
    transaction in which a person buys something that of known description, such 
    as paying fifty dollars today on the basis that he will take 100 kg of rice 
    in six months time. But it is not permissible for this transaction to be 
    uncertain, according to the market price, such that the purchaser may get 
    100 kg or he may get 200 kg. Rather he is not entitled to anything other 
    than what has been agreed upon, which is 100 kg, no matter what changes 
    there are in the price, because the Prophet (blessings and peace of Allah be 
    upon him) said: “Whoever pays in advance for something, then (it should be) 
    for a specified measure and a specified weight, (to be handed over) at a 
    specified time.”

    Narrated by al-Bukhaari (2241), Muslim (1604). 

    Payment in advance here is a salam transaction, but this kind 
    of transaction (salam transaction) is not valid when it comes to selling 
    gold, silver and currency, because the permissibility of trading in these 
    things is subject to the condition that the exchange take place on the spot, 
    and it is not permissible to sell them for deferred payment or payment by 
    instalments. 

    For more information please see the answer to questions no.
    65919 and
    22869

    And Allah knows best.