Debt contract

I am putting together my first debt contract for a real time slave. We have talked extensively about the topic but I still need opinions on the details. We've agreed that the monthly payment should be $250 with interest rates/late fees. I would love anyone's help on this matter while I draft this. What should the interest rate be? What should the late fee be? Is there anything else I should put in? He wants to work towards paying off one of my credit cards which is about $6000.