Is a person's gift to the lender after repaying the debt, with the intention of compensating him for an unseen harm, considered usury?

If you pay off a debt and add something to it out of your own good will, or if you give a gift to the creditor as compensation for a delay, then this is good and there is no harm in it; because of the Prophet, peace and blessings be upon him, who said: "The best among people are those who are best in paying off debts."