What is the religious ruling on a father settling a debt that he guaranteed for his brother who passed away thirty years ago, and without the knowledge of the creditor's heirs? And does the value of the debt change now?

Your uncle's heirs must repay his debt from his estate if he left one. It is commendable for them to repay his debt from their own funds if he did not leave an estate.

Regarding your father (the guarantor): - If his guarantee was a "guarantee of presence" (merely to bring the person), then the guarantee ends with the death of the guaranteed person, and your father is not liable for anything. - If his guarantee was a "guarantee of wealth" (an undertaking to repay the debt), then your father must pay the amount to its rightful owners or their heirs. If they cannot be found after searching, he should give the money in charity on their behalf. If they are found later, they are given the choice between approving the charity or taking their right.

To know how to settle in case of currency or value fluctuation, please refer to Fatwas No. 32964 and No. 34657.