What is the ruling concerning the money paid to the shop in exchange for purchasing gold? And is it obligatory to buy the same initial necklace, or something else of the same value, or should the money be left with him?

What occurred between the questioner and the seller was a rescission of the contract, and the questioner left the price with the seller as a trust to buy other jewelry with it. There is no harm in this, because rescission is a cancellation and not a sale, so there is no need to re-contract. The rescission is valid even if the price is not paid immediately, and some scholars hold that immediate possession is not a condition for rescission. There is no need to revert to the same contract, and it is permissible to buy other jewelry from the shop.