Put what you did with a price attached. On a separate line put a discount for that much money (with reason for discount if you want). So instead of "what you did for free" it's quantified $X discount.
Which is another way of saying "Don't do anything for free" or "There's a big difference between $1 and $0."
If there's a line item you're charging them, and then a credit, it's apparent to everyone that your work has value. If it's just missing... then the value can be forgotten much more easily.
If there's a line item you're charging them, and then a credit, it's apparent to everyone that your work has value. If it's just missing... then the value can be forgotten much more easily.