It sounds to me like you should ask the customer to sign a note, an i.o.u., for the debt, with interest and a payment schedule; if they won't do that, I think you will probably need to sue them to get a judgment. That would be as much protection as you can get; depending on the customer's debts and assets, that might not be much, but at least it will get you a bit closer to the "front of the line" than if you have an unliquidated (legally undetermined) claim when (if) they file.