Adjustments after invoice is paid

I'd like to be able to apply a discount or make a change to an invoice at any point in the process and not just before a payment is made. This would be helpful if someone cancels. I had a wedding client cancel that I want to keep records of, but their final payment is outstanding and I let them out of their contract. I kept their retainer, but I gave them 6 months for another session. It messes up my income records / late payments not to be able to edit that large sum out. The invoice could have a cancellation stamp option with options to refund the retainer or not. Or something along these lines!