What's a good interest and late fee penalty for a seller financed balloon payment?

[AC Cooley]

I have a deal where the buyer (who is an investor) has most of the cash for the purchase but he needs to get $7,000 from his job's saving plan account. It will take 7 to 10 business days for him to get the funds. I need to close soon though. So we agreed to add another $1,500 to his offer price for letting him pay some of the money later. We agreed to a balloon payment of the remaining balance due in 14 business days.

What is a fair but strong penalty to put on the contract? Such as one-time late fee and interest accruing every month?