I'm trying to change a flight from Y to F paid for with cash. I also have a FFC and it looks like I can use it - when I search it gives me the amount of fare difference - so it looks like I can use. But when I go to the checkout page, the fair difference shows as an amount I need to pay and I only have the option to use a credit card or pay in installments.

What am I doing wrong? I don't want to cancel and rebook because that would give me 2 FFCs to deal with.