Is purchasing items in Dirhams, then settling their value later in Egyptian Pounds at a price different from the price on the day of purchase, given the existence of a difference between the buying and selling rates in banks, considered usury (riba)? And is delaying payment of part of the amount a sin?

If your friend bought something for you with his own money, this money is considered a loan, and it must be repaid with an equivalent amount. It is permissible to exchange it for another currency at the time of repayment, provided that you do not separate while there is still something between you; this is based on what was narrated from Ibn Umar, may Allah be pleased with him, that the Prophet, peace and blessings be upon him, said: "There is no harm in taking it at its price on that day, as long as you do not separate while there is still something between you." Repayment must be at the day's exchange rate or less, and it is not permissible to pay more. If there is more than one exchange rate, the creditor has the right for the calculation to be based on the highest selling rate in the banks.