Is buying gold for my fiancée from a store owned by my brother's friend, knowing that my brother will pay for it later (after a week), considered Riba an-Nasi'ah (interest from delay)? And what should I do if it is; should I sell the gold and buy other gold?

The sale and purchase of gold for money require immediate possession (taqabudh) at the session of the contract, based on the hadith: "Gold for gold, like for like, hand to hand; silver for silver, like for like, hand to hand. But if these types differ, then sell however you wish, if it is hand to hand." Accordingly, separation before taking possession invalidates the contract. This can be rectified by returning to the seller, giving him the gold and taking the money, and then conducting a new, valid contract.