Does it fall under the ruling of usury (riba) for a person to sell a gold pound to buy a gold earring from the same seller with its price, then exchange the first earring for another of a higher price from the same seller? And is there expiation for that?

The correct method for selling gold and purchasing new gold in its place requires receiving the money first before buying the new gold. This is to prevent circumvention of selling used gold for new with disparity, and to achieve immediate exchange (taqabud). However, if one buys gold and then wishes to return it and buy different gold, it is permissible to annul the first sale (iqālah) and receive the money, then purchase the new gold with that money or by adding more. This is because iqālah does not stipulate immediate exchange, and it is permissible to exchange (musārafah) for what is a debt if the substitute is taken possession of in the same session.