This is simple, get payment on what was contracted before doing any more. If they want EFGHIJKL then give them a quote on it. This is the professional way to do things.
If you do EFG before getting paid out of some misplaced sense of 'whatever' expect to be told to do HIJKLMNOPQRSTUVWXYZ.
Do whats in the contract, get paid, then negotiate anything extra.
I have been in this situation many times, I don't get into a prolonged dialogue about it, I just demand payment politely and professionally keeping it short and clear. After which I ignore anything that doesn't include a confirmation of payment.
"All that is outside the scope of the work I originally quoted on and completed. I'm happy to negotiate doing it but I'll need to be paid for the existing work first. Please find enclosed a copy of my invoice and arrange payment as soon as you can so that we can move forwards. Kind regards etc,."
I have been told that this will lose me future work, but pragmatically it rarely has, secondly I don't have a use for a non paying problematic client. Freelancers often put up with a lot for the money, but if the money isn't paid in a timely fashion there is no point.
Always remember that as a freelancer you are not in their hierarchy, any pandering to this sort of rubbish weakens your current and future negotiating position. Just entering into a involved dialogue over it costs you time and money and should be avoided.