Is it permissible to buy foreign currency from the central bank at a certain price, then for it to sell it to me at a higher price, provided that these profits are added to the loan repaid in installments, or is this considered usury (riba)?

It is permissible to purchase medical tools in installments from a bank if the bank buys and owns them, then sells them to you at a higher price than the purchase price (Murabaha sale for the one who commands the purchase). It is not permissible for the bank to give you dollars for you to buy what you need and then repay them in another currency after a period; because this is Riba al-Nasi’ah (usury of delay), and the currencies circulated now are treated like usurious commodities, as they have taken the place of gold and silver.