What's the last sentence of debt repayment?

The first sentence is right. You should pay back the money you borrowed. If you don't pay it back, it's unfair to the person who lent you the money. Therefore, it is only natural to pay back debts.

The second sentence is not entirely correct. The debt owed by the father is the son's filial piety to his father. But this is not taken for granted. It is right for a son to pay his father's debts. However, this is not necessary. A son can pay for the money owed by his father because of profligacy and gambling, but he must repent, otherwise it will not be taken for granted.