What is the ruling of Islamic law regarding a debt that was in US dollars, then converted to Syrian Pounds, and upon repayment the creditor demands its return in dollars, while the debtor believes he is obligated to repay it in Syrian Pounds given the appreciation of the dollar?

Your father must repay your uncle the amount in dollars, as long as he borrowed it in dollars, and it does not matter if his need was for Syrian currency. A borrower repays in kind for fungible items, whether their price decreases or increases. And it is from the goodness of repayment that your father returns the amount he received without reduction, for "the best among people are those who are best in paying off debts." Moreover, the principle in repaying debts fixed in a certain currency is by restitution in kind, not by value.