Is it permissible to repay a debt with an increase without a prior condition?

The questioner : Your Eminence, if I borrowed, for example, five hundred riyals from a person as a loan, and after a while I returned six hundred riyals to him of my own accord, increasing it by, for example, one hundred riyals, and he did not ask me for this, does this fall under riba or not, Shaykh?

The Shaykh : Did you hear him say, "If a man lends me five hundred riyals and I give him six hundred at the time of repayment, is this riba or not?" We say it is not riba, because the Messenger of Allah (peace and blessings be upon him) said: Just as it is permissible for me to give him something better in description than what I borrowed, so it is permissible for me to increase the quantity, and there is no difference. For example, if you borrowed a sa' of rice from him, of medium quality, and then gave him a sa' of rice of good quality, is that permissible or not? I am asking you, is it permissible? Good. So if you gave him a sa' and a half, there is no objection, on the condition that it was not stipulated at the time of the loan. If it was stipulated at the time of the loan, then it is not permissible.