Is it permissible to return a sum of money with an added "gift" to the one who offered financial assistance, without a prior agreement, out of fear of falling into usury (riba)?

There is no harm in rewarding a giver by returning the gift and adding to it, because of the Almighty's saying: "Is the reward for good anything but good?" and the Prophet's (peace be upon him) saying: "Whoever gives you a gift, reward him." Similarly, an increase in the repayment of a loan is permissible if it was not stipulated, and the debtor may repay it well, because of the Prophet's (peace be upon him) saying: "May Allah have mercy on a servant who is lenient when he sells, lenient when he buys, lenient when he repays, and lenient when he demands repayment." This is further confirmed by the story of the Prophet (peace be upon him) when he borrowed a young camel and returned a choice quad, saying: "Indeed, the best of people are those who are best in repaying."