Is the debt owed by the father, amounting to 1000 Jordanian Dinars—of which 990 Jordanian Dinars were received—to be repaid with the value of 990 Saudi Riyals (the amount sent), or with the value of 1000 Jordanian Dinars (the originally requested amount)?

Since the contract between your father and his relative was a loan in Jordanian Dinars, the equivalent amount must be repaid in Jordanian Dinars. The difference in its value relative to the Saudi Riyal is not considered. The principle in loans is to repay the equivalent, so currencies are repaid in their like, no matter how their purchasing power changes. It is permissible to agree with the creditor on the day of repayment to pay the debt in a different currency at its exchange rate on that day. It is recommended to appease the creditor with an increase over the due amount as an act of kindness, for the Prophet, peace and blessings be upon him, said: "Indeed, the best of you are those who are best in repaying debts." Repaying the debt on behalf of your father from your own money is considered an act of filial piety towards him after his death.