All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
There is nothing wrong with buying and saving gold coins, and this is not regarded as forbidden hoarding of wealth. A hoarded wealth is that for which the owner fails to pay the due Zakaah, and as long as he pays the Zakaah due on his wealth, it is not regarded as hoarded, regardless of its amount. Allah, The Exalted, Says (what means): {And those who hoard gold and silver and spend it not in the way of Allah – give them tidings of a painful punishment.} [Quran 9:34] An-Nawawi said in Sharh Saheeh Muslim: “The preponderant view that is held by the majority of the scholars is that hoarded wealth is that on which the due Zakaah was not paid. If you pay Zakaah on your wealth, it is not regarded as hoarded, regardless of its amount.” [End of quote]
Also, this does not involve prohibited Gharar (risk and uncertainty in transactions) and has nothing to do with it.
What is important in this regard is to comply with the Sharee‘ah rulings governing the currency exchange and gold selling, such as the fulfillment of the Taqaabudh requirement (reciprocal taking possession of purchased commodity and its monetary equivalent by the buyer and seller respectively) in the transactions and the prohibition of delaying the exchange or receipt when selling currency for other currency or gold (the exchange must be hand to hand).
The same applies to the conditions of immediate hand-to-hand exchange and being of the same amount when selling gold for gold or when selling a given currency for its kind, as well as paying the Zakaah on them, if it becomes due.
Allah Knows best.