Must I repay the loan to my father in Syrian Pounds, the currency in which I received it, or in Saudi Riyals, the currency into which it was converted, knowing that the Syrian Pound has lost its value and there was no prior agreement on the method of repayment?

The borrowed amount is twenty thousand Saudi Riyals, and therefore, what is due for repayment is twenty thousand Saudi Riyals. The fact that you received Syrian Lira is irrelevant, because the loan amount is in Saudi Riyals, and the lender converted it for you based on your request. Thus, the debt is fixed in Saudi Riyals and must be repaid in kind, not by its value on the day of borrowing. The consideration in repaying fixed debts in a certain currency is by its like amount, not by its value, because debts are settled by their equivalents, and it is not permissible to link fixed debts in one's liability to price levels.