I have a problem concerning whether or not I am committing riba in lending some money to a friend of mine. I lent $100 to a friend of mine. Then I told him that he should pay me back at the end of the month and he agreed that he would pay an interest of $30 dollars if he couldn’t pay me back on time. The following month, he still pays me $30 dollars because he couldn’t pay me back the $100. The reason why I charged the interest to which he agreed upon is that he would not delay in paying the money and that he would still remember his debt. So, the question is “is my action in charging him an interest of $30 per month to which he willingly agreed upon for borrowing my $100 worth of money deemed as riba? Thank you for your time.