Is the amount the paternal uncle paid for the dowry and expenses considered a debt on the son that must be repaid, especially since the father had paid the paternal uncle a similar amount, and both the paternal uncle and the father have passed away?

If your father informed you that your paternal uncle paid the dowry and wedding expenses with the intention of recovering them from you, then you must repay your uncle what he spent. If your father gratuitously paid this debt to your uncle, then your responsibility is discharged, and you are not required to repay anything. However, if your father did not give your uncle all that he paid, then you must return the remainder of his due to your uncle's heirs.