Is the debt owed in riyals or in pounds, and how is it to be repaid now, taking into account that a portion of it was already repaid in riyals?

What happened is that you were authorized to purchase gold with a loan, and a loan is repaid with its equivalent. Therefore, your friend is obliged to repay the amount you paid for the gold in Riyals, which is 4100 Riyals. It is permissible at the time of repayment to agree on another currency at the exchange rate of the day of repayment, according to the saying of the Prophet, peace and blessings be upon him: "There is no harm in taking them at their price of the day, as long as you have not parted and there is something between you."