I'm not sure where you can find the fare rules. I seem to remember that you can pull them up from your reservation, but I'm not sure. I don't have any active reservations, so I can't check.

As for the refund, I don't think you get back the money minus the $140. What they do is credit the difference to the next flight you purchase. That has to be done within one year or you lose the whole thing. I am doubtful that you will have any luck disputing the charge with the credit card company. The airline will take the position that they acted within the terms of the agreement and the card issuer will have to agree unless you can positively prove that you are in the right.

I don't know if this applies, but another thing to think about is that the change of you departure date (which I assume is later) may now mean that your return leg is violating the L fare rules. The discount fares often have requirements for a minimum stay or to stay over a weekend. So, you need to make sure that didn't happen with the change you requested.