Are you able to clarify your question? Is he borrowing the �100 a month from you and then you want him to repay it with interest? In that case, you would decide the cost and the interest yourself. For example, if you would have received 2.5% interest by adding the money to your savings account, you would charge this. Or, if you would be charged 16.9% interest for borrowing the money off your credit card, you could then charge him that amount and any other fees you would normally incur. However, if you expantiate a little, I may be able to explain further.