If you cancel a ticket midway through a trip (having already taken at least 1 segment of a multi-segment trip), do you get to keep (and re-use) an amount equal to the price of the segments that you haven't taken? Or does AA calculate the amount available for re-use another way?

I have a long business trip planned for next week and was thinking of taking just the first segment (out of 4), just to get the miles, and would like to cancel the trip after the first segment is taken.

If the ticket was about $400, assuming that each segment costs the same, I would guess I'd keep $300 for re-use (minus a $200 change fee)?

Or is the entire ticket gone once I've used the first segment, or another amount left?

Thanks.